Correct me if I'm wrong, but I can't really find a good quote from the main rules that say you can embark units in reserves

so if anyone can direct me in the right direction (page number maybe?) that would be great.
Assuming that you could though, from what I have read, I agree with what Spetulhu said, because the rules clearly state
"...the unit and the transport will be rolled for together and arrive together."
This indicates to me that the unit simply merges with the transport for the purpous of deployment.
On a more fluff based basis, it could be assumed that the overall comander has simply attached one Valkyrie/Vendetta to each platoon and the Valkyries/Vendettas are flying in a squadron. Because only one lucky squad(or [i]unlucky[i] as the case may be) gets to travle with the Valks the squad is simply detached from the platoon temporarily until the either the battle ends, the valk or the unit is destroyed or the overall comander changes his mind.