If it's anything like the 757/767 then the FMC should continue to cycle way points and provide vertical guidance, so if you're not too far off the lateral path it won't make much difference. Its generally only something you see during the approach phase, so as others have said if you're way off then either open the window and use Vnav Speed or go for a different mode altogether.
It's also a good idea on vectors to extend the centre line, which gives vnav guidance to the next way point regardless of where you are and where you're pointing. It just means that if you're flying perpendicular to the track you'll essentially be flying level until you turn inbound and if you're flying downwind the magenta line will actually be climbing (Which in approach logic would command a level segment.)