Can a unit embark onto a zooming flyer?
No, with an exception made for Necrons. For everyone else it must be in hover mode.
I'm looking at embarking my IG Platoon command squad into a vendetta - I'm assuming that the squad can't embark onto the flyer before the start of the game because it has to deploy with the rest of the platoon, so I'll have to "pick them up" with the vendetta after it enters from reserves.
You can choose to put them in a Valkyrie before the game, the whole platoon doesn't have to deploy together.
I just need to know if the vendetta can enter the board by zooming and pick up the passengers or if it needs to enter by hovering and then pick them up?
The first time your flyer arrives it can hover on to the board -which is worth mentioning because I frequently hear people say they can't. There is small print in a box on page 81 that says "A Flyer with the Hover type must declare whether it is going
to Zoom or Flover before it moves each Movement phase. This
means that, if the Flyer arrives from reserve, you must declare
which type of rnovement it is using before placing it on the board.
If a Flyer is in a squadron, all vehicles in the squadron must choose
the same type of movement."
I think the confusion is because of the rule that says if it leaves the table and comes back on, when it arrives from reserve it can't Hover. "A Flyer that leaves combat airspace must Zoom back on when it returns from Ongoing Reserves, even if it has the Hover type."
Also with regards to disembarking, the IG FAQ states that if the vendetta moves more than 6" the passengers must disembark by grav-chute insertion...I'm assuming this applies when the vendetta is zooming too?
Yes; whatever method it uses to move more than 6" you can force lunatics to grav-chute out of it.