Hey, Codrus, I was at a similar point years ago and I'll show you what can be done. My advice comes from a STRONG two channel slant; if you spend 90% of time listening to two channel and just want the HT to mess with, this is a plan for it to work for you.
Give up trying to get "comparable sound" out of an inexpensive 5.1 setup. It won't happen, and if you think it will, you'll be disappointed. For $1,500 obtaining speakers, electronics, etc. you'll get a functional surround.
Don't get too hung up on the idea that the Vandy's are not terrific HT speakers. So what. Sounds like you're mainly a two channel person but just want HT once in a while. You're not about to dump your beloved two channel speakers to do HT. (But if you are, then that's a whole different discussion) Don't let a HT tail wag a two channel dog.
For now, seriously, I would buy two pair of Insignia 6.5" speakers from Best Buy and use one pair for rears, as well as a single one for center. All four for under $100. I've tried them in my main rig and they sound great - would be very usable for these purposes. You can try them out and if you don't like them, return them! Then put bucks into electronics. The Insignias will mate fine with the Vandersteens; you're not building the worlds hottest HT. Straight up, you'd have to pump a fair bit more into surrounds and you'd defeat the limitations of your budget to look at higher cost surrounds/center.
If you're not an HT fanatic don't worry that your center is not "voice matched" etc. Yes, it's better to have a matched center. The cheapest Vandy one is about $200 used, probably worth the extra $150 IF you'd do a lot of HT. But, if you can't find one, or only would watch a movie once a month don't lose sleep over it. At your price point, you have to perform triage; something's gonna give, speakers or electronics. I'd say speakers should give way to electronics all things considered in this situation. If you think you must have Vandy surrounds you'll end up blowing your budget on the speakers and wind up with crap electronics to stay within budget or way overspend.
You'd be better off selling the Adccom Pre/amp and upgrading to a nice integrated for two channel. There are some fantastic integrateds for under $1,000 used here on Agon. If your Adcom stuff is anywhere near 10-15 years old this is a no brainer upgrade; any one of a dozen newer integrateds, especially tube hybrid integrateds, will sound MUCH better! Make sure to get one with remote, if important to you. The upgrade to integrated will make the Vandy's sound better in stereo and HT modes.
You're absolutely right; you don't need subs - yet. Eventually, If you want to get serious about HT you'll want to look into it.
Find an Outlaw or Rotel used surround receiver to power the surrounds and do processing, and you'll actually be done within budget. Go much beyond these bounds and you WILL be over budget. And did you even consider Interconnects and speaker wire for longer runs? Take pencil to paper for that cost too.
Your goal is too small; Your goal should not be to prevent damaging sound quality in two channel, but to improve two channel while adding HT! It can be done. If you LOVE the idea of upping the two channel quality and having serviceable HT, then follow my directions and you'll be very happy! If over time you are getting into HT more, you'll have opportunity to upgrade the surround speakers.
I'm sure others would say keep the pre/amp and put more into surround speakers and less into electronics for surround. Fine, BUT there goes your two channel upgrade!