A Landraider can carry ten Astartes, take out it's Lascannons and associated ephemera and it's transport capacity increases by 50% (the numbers vary according to which edition of which Codex you read, but it's a good baseline) even when replacing the LC's with other weapon systems. This is specifically stated. Lascannon Generators/capacitors* are bulky enough that (assuming one per sponson) they occupy the space taken up by 2.5 Astartes in armour.
Apply that basic number to a Vendetta with it's three TL LC's and you have to lose 7.5 guardsmen to make the gunship viable. Even if you could jam the Guardsmen into the 4.5 capacity slots left the additional weight on an aircraft is far more crucial than on a land-bound vehicle, affecting the vehicle's operational range for more acutely. All of the other Valkyrie/vendetta armaments (bar the multilaser) are dropped or launched and thus can be heavy as they (or at least a majority of their weight) only be carried half of the distance.
*I say Capacitors as it strikes me as odd that a Landraider with a plasma reactor engine would need to actually have separate generators for it's energy weapons, you would just need to syphon off enough energy from the reactor when you needed to take a shot, a capacitor would allow you to "hold" a shot/salvo's worth so you could do it on the trot.