Trigger, I follow your thinking on the squadron designator, and allow me to offer my suggestions.
The "L" in your VMLA (assuming you took it along the lines of the HML/A) would connotate a "light" lift capability. Marine skid squadrons have an "L" designator because the Huey can be utilized in a light-lift role. I'm assuming your armed tiltrotors won't be carrying much cargo, so we'll leave the "L" out.
Where does that leave us? VMA? Sounds like a Harrier squadron, so maybe we'd need a new letter to denote the tiltrotor classification. That's where I'm slightly confused.
The Osprey training squadron at New River was previously VMMT-204. Now that "T" was for training, not tiltrotor, I'm 95% sure. Later, the unit was redesignated as VMX-22. Not sure if the "X" is classifying the tiltrotor designation, or the training mission.
Maybe ridleusmc or supercobra, who are a little closer to the active-duty buzz than I am, can throw out a suggestion.
VMXA-???
For numbers, maybe you convert an existing Harrier, or God forbid, a skid squadron to the new airframe. Better yet, pretend the Corps got a budget windfall and just formed brand-new squadrons.....