I think it's a pretty common issue with older babies. They do get set into their little routines. I took away the bottle before 12 months (my son was able to use a cup between 6 and 7 months), because it's like they hit a certain age, and suddenly everything is a battle. A great way to try this at least the next time around for all the moms out there is to measure out the formula/breast milk into a bottle, then pour into a cup. If they get used to drinking formula out of a cup, the milk transition is easier. I think there's also a taste issue. Let's face it--milk does not taste like formula. I also give my son soy, because he will not eat meat, so he needs the protein. Soy milk also has B-vitamins, which is important for brain development. However, I do NOT recommend the VERY VANILLA. Check the sugar content; it's very high. Regular vanilla has much less sugar and is just as good.
Don't worry about your son dehydrating. First, it's not hot enough right now to affect him, and by summer, with persistance on your part, he'll be drinking the milk. Secondly, most foods have a water content, especially fruits and veggies. Last, if he willingly drinks water, then he'll be fine. I do recommend watering down that juice though. I do half water and half juice for my son, since he prefers it over plain water.