Wouldn't want to advise on the pre-booking. We've been there without booking anything and it does give you great flexibility and the ability to haggle on the spot. However, that wasn't on 12 December and as everyone says, Christmas will be very, very busy and you may end up somewhere a bit grotty. Peace of mind has a value and it may be worth getting something booked.
Don't worry about the veggie food. I'm vegetarian and the food is a highlight of the holiday. I've never holidayed anywhere where the veggie food is as good as Goa - virtually all restaurants have a vegetarian section on the menu, and some restaurants are exclusively vegetarian. Sure, a lot of it is spicy but there's plenty that isn't, and you can wean yourself onto various curries as you progress. Staff are very helpful and will explain dishes and degrees of spicyness.
Have a good look at the restaurant recommendation thread as it develops this season (and take a look at last years). Worth a try in Candolim are Soulyman's Kitchen, After Seven, Flambe, Roma (Italian) and there is a purely vegetarian cafe/restaurant in the centre of Calangute called the Plantain Leaf, which I like. It's very Indian and you'd have to be careful what you order to keep the spice under control! Also, definitely worth a visit is Bean Me Up - a brilliant purely vegetarian restaurant in lovely garden surroundings in Anjuna. Think about lunch there after a trip round the weekly Anjuna market.
Have a great time, whatever!
