"not chocolate" ;-)

I guess you could try saying that you'd like a "plain" milkshake, and you'd probably end up with vanilla.

Well, you have two options:

1) Point at the menu. (I do this!)

2) Learn the V and L sounds in English.  I realise that it's difficult because some English sounds don't exist in Japanese (such as V!), but ask a friend to show you, then practise. That's really the best and easiest way. :)