I am assuming for wakeboarding/wakeskating.
The straightline Sumo 600's fit perfectly in the lockers of your VLX, plus the inside diameter of the ports on the sac are 1". I have the fly high 750's in my A22 (yes, different lockers than yours but very similar) and I tested a Sumo 600 in one of my lockers and the actual weight appeared to be identical. The 600 fills completely without putting any pressure on the engine dividers or seat back and access panels. Wakemakers sells them and they are the same price as the 750's, but in my opinion they are better built and the locations of the ports are better. Sumo also has an extra port that is designed to bleed air but also takes the tsunami pump.
If I was doing my Axis over again, I would go with the Sumo 600's.
Then for your bow, if you have bow ballast, you can piggy back into a bag system that goes under your bow seats. Sorry, I do not know off hand what the bags malibu guys are using. you can search it on the malibucrew forums though.
If you are going to surf, go with the Sumo 900's or flyhigh 1100's. in the rear lockers.