As a former GM and recruitment officer for 7 years of a heroic 25 man guild I can say this to you:
1. Apply now
Why should they take you now?
Well they won't lose anything if you are already in the guild. You can begin to feel at home in your new guild already. They can atleast socialise already with you. But don't expect a lot of socialisation either. When the guild is still raiding, they won't have time AT ALL to turn around and pay attention to you. That starts only from the moment WoD hits. Don't ask for a raidspot during this expansion. Just tell them that you'll do whatever it takes to be raidready once WoD hits within the allotted time the new guild asks of you. My guild usually asked for 1 week to 1.5 weeks to get raidready. Some guilds are far more lenient. (ask them what this time is)
2. Be sure to tell them that you are a Vanilla player
It doesn't matter if you are a Vanilla player. It doesn't show anything really. Vanilla players could just as well be new players. The game has changed so much. But you do already grasp the basics.
3. Be sure to tell them that you are willing to read and watch strategies (just like you did before?)
4. Be absolutely sure that you want to raid again
Nothing pissed me off more then having a recruit being full of energy at first, you gear them up in the new tier and POOF, gone they are.
Turns out they just wanted that feeling back they once had in Vanilla. And they couldn't get it. No shit sherlock, you won't ever get that feeling back.
Turns out they felt that raiding is too much work. They expected the game to not be so demanding. Dude have you played Vanilla?
5. Write a decent apply with proper words and do not use XD or >.< etc. Use words.
Do not hesitate to write a long apply. Think about what you want to say. Most people just write an apply in 5 min. I reject them in less time I can assure you. The ones that actually took effort in writing their apply, I take far more serious. I could even think: Hey this guy must have spend atleast a few hours on this. Maybe he'll have the same drive in raiding too?
6. If you just had a baby and want to apply to a heroic progression guild, do not bother!!!!
I cannot say this enough. Employers usually hate it when their female applicants are pregnant, guildmasters do not like it when applicants (male or female) just gotten a baby. You will not have time enough for raiding if you have a baby, period. Or you must be looking for a divorce. You can say all you want about: well my wife and me have it all worked out. Yeah I am sure that after one year of raiding the significant other still feels it was a good deal. And who ends up on the short end of the stick? Yes the guild you are in. Casual guilds? No problem.
7. Be prepared and committed in leveling asap and gearing asap when WoD hits
I cannot stress this enough. I once as recruitment officer took in 20 people before a new expansion came. I explained to them what was expected of them. Three of the twenty made it to be a proper applicant. The rest was still busy leveling after 1.5 weeks. Some (I checked on them) were still not maxlevel 1 month after the new expansion launched. You want proper gems (not the fucking cheap ones) in your newly acquired heroic 5 man blue gear and enchants. You need to have researched what the best talents are for whatever fight is coming. You have proper glyphs. And you know how to play properly instead of being a LFR schmuck.
Oh look it's you again, the most boring person on the internet - i wish you would stop with this line "As a former GM and recruitment officer for 7 years of a heroic 25 man guild"