Enhancing garlic and tomatoes on toast
When you mention that you've specifically cooked it down until the onions are brown and the tomatos have lost most of their liquid, I'd say that you might be near a jam  ... and there are plenty of recipes out there for tomato &amp; onion jam.  Most call for adding a bit of extra acid to it and/or extra sugar.

If you search for 'crostini con pomodori e cipolla' (Italian for "toast with tomato and onion"), most of the recipes have some other added salt component -- anchovies or hard cheeses were common ... but you specifically mentioned vegan.

If it were me, I'd lean towards olives -- they'd hit both the acid and salty notes.  Or maybe pickled hot peppers.  Or even just a pinch or two of pepper flakes.  

As for the problem of it being too dry -- just cook it less.  So long as the bread's toasted, and you're eating it shortly after assembly, a little extra moisture won't hurt.  You can adjust the cut of the toast (if you're slicing it yourself), or cut it in half, so that it's less likely to flex if it starts to sog, or add some barrier (eg, olive oil) to the bread first so the liquid can't soak in.

You could also try roasting the tomatoes to dry them out and help activate their pectin to gel the remaining liquid.  (or add extra pectin)