A Couple lighting Qs

[Rusty Trichome]

First of all I know that reducing light hours to under 12 is supposed to induce flowering but does that mean it halts the vegatation (stage)?

Yes. But they (the plants) will stretch early in flower, then they'll fill-in if given ample time to mature.

Second, lets say some plants have been growing about 4 months. The secondary branches seem to be growing very slowly. The plants have about 10% red hairs at this point. Would this be the right time to reduce lighting hours?

Hairs (pistils) are not an indicator of plant maturity, the trichomes are.
How tall is the plant, what size container are they in, how much headroom do you have for when they stretch? Is it an indica dom, or sativa dominate strain?