How big is the tank he's in? Does he seem to have anything wrong with his body and eyes?
If he's blind then it would show in his eyes, they would be cloudy (in my experience). And if he is then it's not something to worry about. I owned a one eyed Betta, he was rescued from PetSmart- one of his eyes was missing and he was partly blind in the other. I got him a feeding ring and had to feed him flakes, as the pellets sunk too quickly for him to eat them.
Like mutestar said, try and see how he responds to light, and a bright light on the fish should show if he has any external issues that your eyes can't pick up in regular lightning..
If he's not eating don't worry too much about it, Bettas can go a month without food and live. I would go without feeding him for acouple of days then try some frozen Bloodworms to tempt him. You can buy frozen Bloodworms, Brine Shrimp and Daphnia at PetSmart and PetCo with no problem. Brine Shrimp and Daphina especially are mainly used to flush out the fishs' system as it acts like a natural laxative. Therefore it shouldn't be fed to your fish constantly because it could cause internal problems. Bloodworms are, of course, used as a treat and like Brine Shrimp they should only be fed like so, two to three times a week.
In the mean time you can try getting him some API Stress Coat, you should be able to find that at your LFS, it's a water conditioner and is great at calming fish down. Since you were gone for vacation he may not have gotten any attention and may have been upset about it. Especially if you cleaned his tank right away, imagine being left to yourself for a week (even with friends feeding him) and then all of a sudden the water being changed, it might have freaked him out a bit.
If he has somehow lost his sight it's not something to fret about, Bettas are very smart and can live just fine without their sight. You'll want to not move his decor as it can confuse and scare him and feed him in the same spot every day, or whenever it is you feed him.
Also, if his eyes are cloudy: 1 gallon QT tank, Stress Coat, 1 tsp aquarium salt (it's 1 tsp per gallon), and some API Fungus Cure- or any freshwater medication that clears cloudy eye. You can try with just the Stress Coat and Aquarium Salt for a week first before using any medications. You don't want to use AQS for any longer then 7-10 days at a time.