Is a Wusthof 7-inch santoku knife adequate for my vegetarian needs? I worry about cutting squash and large produce with a 7-inch knife.

A Santoku is a useful knife to have in your tool box but a 9" chef's knife is better still. Plumbing the mysteries of Japanese knives is a sweet science.

I agree that you might want a knife a little longer, but as big pan says, a very good idea to test drive a few in a store. Don't feel pressured into buying anything on the spot. You may find that with a decent sharpening steel or stone, you could use shorter, less expensive knives or paring knives for chopping up smaller vegetables. If I recall, Cook's Illustrated had a knife comparison a year or two back and one of the best deals was a Victorinox that cost about $30 and possibly less.

I have a Calphalon 7-inch nakiri, and I use it for everything -- even heavy duty squashes. It costs a bit more than a santoku, but it is worth it. Wusthof makes both types, if you need to compare prices.

Most quality cook shops and/or knife shops will let you test-drive a knife in the store. I would go for one a bit longer but also make sure to hold it and feel how comfortable it is for your hand size and if you like the weight or not.
If your heart is set on the Wusthof and you find the larger knife too heavy for you, then consider a lighter weight quality knife, like Global.

I have a 7" Shun santoku. I have to admit it may have been smart to get the larger. I have a vegetable cleaver and a meat cleaver which I grab when the task feels too much for my Shun.