If they say that tell them to take the new tires off, and put a set of trash tires on and you'll take your business elsewhere. If they dont want to do that, inform them that you will be contacting corporate Sears, the Better Business Bureau, and your personal lawyer.
Be tough with these guys!
When you walk in, tell the guy at the counter:
"You mounted new tires on my Lexus yesterday (or whenever), and now I am experiencing a vibration that I did not experience before, may I talk to the manager."
When the manager comes out
"Hello sir, I am (insert your name here and shake his hand), your shop put new tires on my Lexus and now I'm experiencing a vibration problem that I had not experienced before. Here are the keys and the wheel lock, its parked out front, I'd appreciate if you could make it ride like it did before I brought it here, I'll be in the waiting room"
Then, walk off and go wait in the waiting room. Stay in control, these guys only get you when THEY gain control, which they never should do.
For what its worth, I believe that Sears offers lifetime rotation and balancing on the tires they sell. I only bought tires from Sears once for my dad's LS400 and I'll never do it again. Every week he or I had to take it back over for a balance, they never did do it right. Maybe they'll clear it up though.
And about the alignment. While its probably not causing the vibration, I do reccomend that you let them do a full 4 wheel alignment whenever they install new tires. Just because tires are a big investment, the alignment is most likely off at least a little and an alignment helps promote good tire wear.
If the guy comes out and says some bleep about it being the alignment inform him
"The only way it could be the alignment is if you all caused it to be misaligned when you installed the tires, because when I brought it in here it was straight, so if you want to do an alignment its going to have to either be free, or we're going to have to split it" If he doesn't want to do an alignment, inform him that you'll take your car and your keys and the rest of your business elsewhere. Also inform him that he'll be contacted by his regional manager and the Better Business Bureau for the purpose of greivance mediation.
Stay firm, dont buy their crap. Don't get angry but dont get happy either. Be indifferent, you dont care what they say or think, you just want them to do the work right.
As for the Lexus dealer, no you dont have to service it at the Lexus dealer, but you dont want to service it at a place like Sears either. You want to find a GOOD independent shop somewhere with an owner and mechanics that care about you and their work. Easy to find.