So as far as I know, the current rule is you can only spend glimmer you get in the run, but there isn't a way to track how much glimmer a single character has earned. For this reason shouldn't buying anything from vendors just be banned?

You just have to make a note of how much you start your run with and make sure you don't drop below that amount.

if you watch any of my new game runs i call out how much glimmer i have each run and watch how much i use in the run