If I use Roundup to keep weeds and grass out of the irrigation ditches that bring water to my vegetable garden, will it end up washing downstream and killing my vegetables? Also, will it harm my vegetables if I use Roundup on weeds growing in the garden if I'm careful not to get any on the vegetables themselves?
The ETOXNET (Extension Toxology Network) page at Cornell for glyphosate suggests it binds quickly to soil so that run off problems tend to be minor (2% being quoted as typical). Note however you are talking about irrigation ditches, so I would expect run off to be higher - although perhaps not by much if you treat the ditch whilst dry and a few days before rain.
Although glyphosate is the active ingredient of Roundup, much of the animal toxicity comes from surfactants used in the Roundup mixture - so a lot of the official toxicity reports tend to under-report the toxicity of Roundup (Monsanto have only submitted glyphosate for testing and not production Roundup mixes).