Hey Jan,
after take off VNAV (when engaging) bugs for V2 + 20 knots.
The real plane however does not add 20 knots to V2,
it adds 20 knots to the MCP speed, that has been set manually by the pilot.
This is why the pilot is supposed to set V2 in the MCP panel,
because if he doesn't, VNAV will indeed bug for a false speed.
In Aerofly the initial MCP speed is irrelevant.
Greetings