I prefer Microgyros ESC over Mtroniks- less expensive and better specification, and they also have some nice sub friendly features like a programmable 2 second built in throttle failsafe.
I'm perplexed by this problem, owning an FC16 myself I know there is nothing unusual about this radio, unlike some of the later Futaba sets which don't conform to the older PPM standards and can cause issues with some third party kit.
You can purchase high current linear BEC's. Jeti do some nice models, but they're quite expensive and bulky when compared with the switch-mode models, as you need a chunky heatsink. Course on a sub, if you were to make one of your end caps from metal, you have a fantastic heatsink that will get rid of any amount of heat.
You can also make your own up quite cheaply, but that requires a little electronics knowledge. the BEC's fitted to the majority of ESC's are generally good for about 1-1.5A, so at 5 volts you're looking at a maximum of 7.5watts. Most bog standarad servos draw between 1-2 watts under normal duty, although they can draw up to twice that if stalled or labouring heavily.
In the case of your vanguard, although you have four servos hanging off the BEC, three of them are fairly low duty, the rear hydroplane servo will be the busy bee coupled up to a leveller. You should be okay so long as you haven't fitted anything fancy like digital servos, which are thirsty little critters.