Canon doesn't define how many units any faction has. It's intentionally nebulous.
How to use the MUL, if you use the MUL at all, is up to you. If the group says "100% MUL" and you want to take a Vulture and MUL says your faction doesn't have it, that's the rules for your group and you don't get a Vulture. If the group says screw the MUL, take what you want, that's the rule for your group. Take your Vulture. Or anywhere in between. (Or use non-MUL availability: use RATs, use Spotlight On defined forces, the Force Manual availability, etc).
Rules and canon are separate. Machine guns don't canonically miss because a target is exactly 91 meters away. Game rules say it does. Faction availability is the same. Canon is too nebulous for a rule. Any faction availability rules are not canon defining.