Glad to hear that you're rethinking the vases!! Your bettas will surely thank you for it in the long run. :)
Ah, the vases are even a little smaller then I thought...its a good thing you're seriously considering another option! Even for the single fish alone thats a pretty small space, especially being all vertical, long-term....generally 2.5 gallons/about 9-10 liters per fish is a good minimum long-term size to shoot for.
For the time being if you can't take back/rehome the snails/upgrade to something much larger within the next couple of days you'll want to start doing some rather frequent water changes since ammonia is going to start building up real quick in such a small space.....I'd say just go for a 100% every other day, possibly some quick 50% or so changes in between depending on how large these snails are. You'll want to really keep on top of the ammonia while they're in those vases so you don't end up with some serious issues before you get a chance to move them out.
And lets see, just to reiterate whats already been said...sorry if I get a tad repetitive, RedCassette gave some great and advice there...and to add a little more...
+1 about the divided 10 gallon/38 liter. You could go as small as 5 gallons/19 liters with a tank divided in half, but I'd say for a beginner a 10 gallon(split in half in this case)is a great place to start; theres quite a bit more room to learn and work with.
Check around local thrift stores and maybe some places online to see about cheaper used tanks. You can sometimes fine some fantastic deals on stuff lightly used. Also if you're somewhere in the United States, Petco is having their $1 per gallon sale at the moment I hear....meaning you could get a plain ol' 10 gallon for $10. Just some tank suggestions....
As for the divider itself, I agree about making your own craft mesh divider. I can promise that it will be far more stable and a LOT cheaper then anything you can find to buy pre-made.
Here is a great tutorial about how to make them: http://www.tropicalfishkeeping.com/d...ividers-21866/
I do have to respectfully disagree about the opaqueness.....while that is true with many fish, that they will eventually get used to one another, it isn't with every betta. Some can get very, very upset and stressed over seeing their neighbor too much regardless of time spent next to them, even with a somewhat opaque divider of some sorts between them. It all depends on the fish, though personally I think its better safe then sorry.
Now, what I do with my dividers is follow the tutorial, but instead I use two sheets of craft mesh and double-up, then do a little more exact measuring and trimming to make it fit snugly without too much of a bend. This make the divider far more opaque, my boys hardly ever noticed each other, if they even did at all, from the very beginning with these types of dividers and it also makes them just that much more straight and stable.
Another great technique with the divider if you end up going with a 10+ gallon is to separate the two pieces of mesh in the back and stick your filter and heater in between. That way you have just a tad bit of space between each fish in case one gets super feisty and finds some small area he can squeeze over the divider(on top of the fact that they of course can't really see one another), it sort of naturally softens the water flow on both sides without having to actually baffle the filter(unless you end up with a very strong one), gives a little more even water flow/heat distribution, and you don't run the risk of fins getting shredded in the filter intake without having to bother baffling that as well. Its a pretty awesome way to do it, I wish I had set my own 10 gallon up that way.
Now! Just to go over baffling the filter in more detail in case you don't end up going that route....
Filter sponge is going to be your best friend in this case(its really great stuff....I'd even suggest tossing away your filter cartridge with the carbon in it and just stuffing your filter with filter sponge. Carbon is unnecessary, and the floss on the cartridges wears out really quick.....sponge will give you a lot more surface area for Beneficial Bacteria(I'll elaborate a little later) to grow and stick plus it won't break down).
For the filter intake to help prevent fins from getting caught and shredded, you can either take a little piece of sponge and stick it inside, or get something like this: http://cdn1.bigcommerce.com/server32....1280.1280.jpg
to stick over it. For the outflow, like Red Cassette suggested, a nice square piece of filter sponge secured with a rubber band, some fishing line/cotton string, or a non-metal hair band over the outflow should do the trick there, though first baffle the intake and see what the water flow is like. Often once that is baffled the water flow slows significantly, IME.
Almost done, I promise!
Now, lets see....onto the matter of cycling!
If you're looking for something large enough to divide, you are going to want to filter and cycle this tank. While Betta fish themselves don't absolutely NEED a filter, a larger setup does require one if only to establish a cycle to help cut down on maintenance....a 100% water change in a 10 gallon is quite the pain, let me tell you.
Now, establishing a cycle is a bit of a process....not quite as simple as letting the tank run for a few days before adding anything like most pet stores will tell you. Now, so I don't go on and on and on with the explanation, here are some great threads to look over that will give you a bit of an insight on cycling and how it works, though do as much research as you can! Google can turn up some fantastic results on the subject as well.
It sounds a little complicated at first, but it starts to make sense after a little while, I promise.
And lets see....besides all that, I think its just the thing about the aging water that I'd like to add on to. While aging you water does help a tad, really its not going to remove ALL the harmful nastiest in there I'm afraid. What you'll want is to pick up a good water conditioner like Seachem Prime. Prime basically removes just about anything potentially harmful in your water including heavy metals and, unlike most conditioners, it will lock and detoxify ammonia for 24-28 hours which is very helpful during a fish-in cycling process. Another great thing about Prime is that its fairly concentrated; just a couple drops treats a gallon(though it is safe at about 5 times the dosage I believe, so don't panic if you over-dose a little!), so you're going to buy less of it over time then other conditioners as well.
And....I do believe thats it for now! I hope I was able to help some.....it is morning here, and I'm still on my first cup of tea so I apologize if my wording is a little off....and please, feel free to ask ANY more questions that come to mind! Thats what we're here for. :)