Yes, any signal that is not a speaker signal should be safe. Headphone signals are basically speaker signals, although as mentioned before, they are so weak so they probably wouldn't damage anything. They are likely to distort or have humoungous amounts of noise, though. Or just generally sound like shite.
Drum triggers usually output a signal that is only understandable by the speciel brain they are designed to work with. Ther may be drumtriggers that do midi directly, but nothing you could input into the VS, obviously. The puropse of a drum trigger is to take a sound (the drum) and make a non-sound signal (the trig) of it.
I don't know if you can sync two VS-1880.