Let's see if I can make this fairly simple.
When you get promoted to Major you will find out if you are an IDE in-residence 'Select' or not. If you are a 'select' you will go to IDE sometime before your LtCol board. That IDE could be ACSC in residence, or AFIT, or foreign IDE, or go to a civilian school and get a masters.
If you are not a 'select' you will have 3 'looks' to get picked up for an 'in-residence' slot. Once a year, and I haven't really heard of anyone getting picked up until their last look. Same 'choice' of school possibilities. You will know when your 1st, 2nd, and 3rd look is. Either way (as a select or if you get picked up after the fact) you will submit your preferences for school and AFPC will send you somewhere in a process similar to, but seperate from, the VML.
If you want to get an advanced degree from AFIT you have to apply to AFIT BEFORE they match you to a school. It's an easy process but you need to do a GRE. I personally think if you are a non-select and trying to get picked up for IDE it helps to be 'pre-accepted' to AFIT so the IDE board can see that. I'll let you know how that works out in a few months, since I'm on my last look.
So right now, you are a little ahead of the timeline as far as looking for the AF to send you to school ... hang on until yer a Major.
As far as doing a masters now, etc. .... If you do well it shows that you have the ability, drive, and time management skills to handle IDE in the future and should be a feather in your cap when you're getting considered for IDE in the future. Once you are a major-select you can start knocking out IDE in correspondance. I recommend the ACSC/Masters DL program. You get ACSC done and a masters in a little over a year entirely online through AU. It's an easily manageable program and then, hopefully, if you get picked up for school 'they' won't send you to ACSC again and you can avoid Maxwell. If you're a 'select' through the promotion then sit back and relax ... they'll send you somewhere.
/ramble off