Should I even worry about farming marks if I'm not level 20 yet?
Eh, I got about 6 or 7 engrams in about an hour total of farming that spot, so it wasn't dumb for me. Much faster than replaying missions over and over.
I don't remember even getting any marks prior to level 20. Just focus on getting to 20, then switch to marks until you get a full set of purple gear. How you do the purple gear will be up to you.
The vanguard uses vanguard marks and rep.
The Crucible vendors use crucible rep and crucible marks.
the faction vendors use all rep earned when you are wearing their class item, in other words all rep from bounties, events etc goes to them and not the other two. The faction reps use crucible marks.
Vanguard Playlist and bounties (PVE) will always be Vanguard. there is no way to switch them to crucible.
Crucible only awards crucible marks. There is no way to switch them to vanguard.
I hope this helps.