737NG VNAV armed on ground.
 

I've just seen a video on youtube of a 737-800 departing with VNAV armed prior to the takeoff roll.

Has this always been possible, or does it depend on whether you have a Collins vs Honeywell MCP?

With VNAV armed, when does it engage?

Only since FMC version 10.8a. It engages above 400' RA.

Not correct, it was available before 10.8, but some older FCC's would engage rather than arm it. Boeing issued a bulletin on arming VNAV with 10.7 in 2007.