This article is pretty thorough - from a guy who's been there and done that.
DO NOT USE A HELICOIL. PERIOD
Didn't realize you had aluminum heads when recommending the insert - or else would have said DEFINITELY do not use a helicoil. They are steel - your heads are aluminum. THE HELICOILS WILL BLOW OUT QUICKLY - GUARANTEED. I did that once (previous posting) - and the engine was gone in less than two months - junked. And I had taken the 'heads' off the engine to do all the work - so it wasn't debris that killed it.
Anyways, here's the article - the guy also talks about NOT using a helicoil for repair.
Very thorough article (and he does use the vacuum method you were talking about) -and covers a WHOLE lot more that neither micro nor myself had thought about.
It IS going to be more expensive than the $35 tap - but it will be worth it. This is the way I would go in your boat.
http://www.autoserviceprofessional.com/article/91470/pro-level-spark-plug-thread-repair-made-easy