Can I use the white from one switch, but the black from another?
You either have to replace the Vent wire with 14-3 AWG wire, or run a second 14-2 wire from the existing light to the vent. Assuming both switch circuits are from the same feed, you only need 3 wires to the vent. All the whites get tied to the white neutral of the feed. You could use the black (or Red) to go to the light - fan, and the Red (or black) to go to the night light. 

This quick schem shows the connection with adding one 14-2 cable from the existing light to the Vent. 

M-L-N can be switched around, depending on which functions go with the shared switch. I assumed the night light gets its own switch.