737NG VNAV armed on ground.

[TotalBeginner]

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?

[Sciolistes]

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

[RTO]

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.