Maybe I'm a little late on the scene here, but I found the cheapest possible to make a Vendetta with parts that every guard player has in his bits box.
You'll need:
A Valkyrie
6 Lascannons from the heavy weapon sprue
2 of the short armor plates from the old
IG tank sprue
knife
glue
For the nose mounted cannons:
1. Cut 2 Lascannon barrels just before the coils
2. Cut the multilaser that comes with the Valk off, position barrels like so:
For the wing mounted cannons:
1. File the sides of the cannons that are going to be glued together (will make sense in a minute)
2. Use the armor plate as a rack, gluing it on top of the twin lascannons. It should turn out like this:
3. Glue or magnatize lascannons under wings
Finished product, sorry for the bad quality, all i have is a cellphone camera:
Since people seem to be reading this tutorial still, here's a much better picture of the completed model: