Sanderxpander
This is kind of useless advice at this point but whenever I track vocals I prep like six to eight tracks so while we're working if they have a harmony idea or ad lib I can quickly switch and name the new track to whatever they're doing.
This ^^^ is great advise. I do the same thing.
To the OP: When a harmony line or add lib comes in to your head, there's no time to fiddle around with a take lane. Keep other tracks prepped and ready to go as SanderXpander suggests.
It the fastest way I know of. typically 4 clicks.
(turn off input echo+record on existing Vox Trk 1/ Turn on input echo+record on prepped Vox Trk 2).
I am not afraid to admit it, but I am not really a huge fan of Take lanes, not because of their functionality but more because of the "Perfect take" aspect of it. Needling vocal lines to death searching for the best take can suck the inspiration right out of a mix.
With that said, there are times I rely on Take lanes. On difficult passages, you know the type of things...
"If you cant do it in one take then you shouldn't do it at all".

Oh ya im guilty of that.
It is a great tool but much like Melodyne can be over done.
take SanderXpanders advise though, Prep some extra Vocal tracks instead of Take lanes.
Its basically the still the same amount of CPU being used (run a send to a bus for FX

) And easier to manage in my opinion.