For what it's worth, the Compadre runs at full sensitivity, all metal until you move into the disc settings where it still is at full sensitivity. If you were asking this question as to picking up the 5 3/4 coil for the Vaq and did not have the Compadre with that combination, I'd say go for it. Being as you already have the Compadre and Vaq both, if it were me, I'd keep both (always good to have some sort of back up) and do just like you're doing. I'm thinking the Vaq might get a bit more depth with the 5 3/4 coil, but probably not enough to be worth the extra expense. The Compadre's are such gems for what they are, when you consider the cost of the 5 3/4 coil for the Vaq versus the cost of the Compadre which is a complete detector plus that size coil the cost to add that coil to your Vaq doesn't compute too well. My two bits.
BB.