Exeggcute by itself is an annoying foe. I went tree-banging in SS and would drop it to half, but once I'm put to sleep and it's doing a Leech Seed or something, and I can't do anything for five turns, it's fully recovered and I'm saying dirty things. LMAO
Yes, all Hidden abilities are only obtainable via Dream world, or breeding a female Dream world pokemon. Some of the Hidden abilities are very useful, like exceggcute's Harvest ability, which in combination with sunny day and sitrus berry means he can recover 1/3 of his health every turn and still attack
Drought is the Hidden Dream world ability for vulpix, so unless you get one from dream world it can't have drought. It just takes 10,000+ dream points to get one and i don't feel like waiting that long lol
I could definitely go Vulpix hunting and seek a Drought ability, but I can't guarantee it. If the ability is only granted upon evolving with the Fire Stone, I could test them out and then proceed to trade you one that can be evolved with said Fire Stone. I personally am not fussy about abilities myself, but if drought is what you want, it can be done. Have you considered that it's the Hidden Ability, however, and not the primary one?
This all assumes I have a Fire Stone. I might have used mine. I'll have to rely on dust spots since I don't have a vibrant Black City.