Vegapunk doesn't have much of a choice, it's either that or he dies (especially now that his laboratory/resources will probably get destroyed or seized and the WG + Judge/Caesar want his head). Morgans apparently doesn't bow to anyone, but I wonder if he'll be cornered someday and be forced to join Luffy. It's a possibility.
If both work for Luffy, then he'll have the strongest force among all the Yonko probably, considering that he already has so many powerful allies. I'd say that he would be even stronger than Roger or WB, all allies counted.