"When I capture video shot in a 16:9 aspect ratio, Will
Vegas and Vidcap display it in a 16:9 preview window? If
so, how do I get that window up?"

You need to set up a 16:9 project, if DV use the NTSC DV
widescreen project template.

"to do letterboxing for a 4:3 display, I've read
here taht you suggest cropping. how Do i make sure my crop
is exactly the right ratio?"

The crop tool can use the project aspect ratio (in this
case 4:3) so you can selectively crop your wide screen to
fit- you can even reframe the shot as the scene progesses
if you keyframe in event pan/crop