What is the maximum percentage of vodka that can be added to an 'adult' sorbet and still retain its frozen consistency?

Sorbet, with or without alcohol of any kind, is much more a matter of chemistry than ice cream, and also very much worth the effort.

Too scientific for me - I'd make a nice fruit sorbet and serve a shot of vodka on the side.

I concur with drbabs' ratio and add that you use a lower proof vodka such as 40% which is 80 proof. I have an 80 proof bottle of homemade limoncello in the freezer that keeps in a slushy state and is more than 2/3 vodka; ... adding 1/2 cup to a quart for your sorbet should be good.

If you don't have sacchrometer, dash into a home brew store and pick one up. They cost around $10. Your sorbet solution wants to measure between 12 and 15 degrees baumé. Go ahead and add your vodka (4 ounces sounds good, as DrB says). Add lemon juice to balance it and achieve the 12-15 degree range. Sounds very good by the way.

David Lebovitz adds 1/2 cup per quart when he's making sorbet, but I think that's just to keep it soft. You could do that and pour more vodka over it before serving if you're going for an alcohol heavy sorbet.