The international organization (ICAO) recognizes Vmca as the official acronym and yes "a" whilst airborne.
Now if you really like letters here is a quote from AC 25-7B which is the guide to certifying your Part 25 aircraft.
23. Minimum Control Speed - § 25.149
Section 25.149 defines requirements for minimum control speeds duringtakeoff climb (VMC), during takeoff ground roll (VMCG), and during approach and landing (VMCLand VMCL-2). The VMC (commonly referred to as VMCA) requirements are specified in§ 25.149(a), (b), (c) and (d); the VMCG requirements are described in § 25.149(e); and the VMCLand VMCL-2 requirements are covered in § 25.149(f), (g) and (h). Section 25.149(a) states that“...the method used to simulate critical engine failure must represent the most critical mode ofpowerplant failure with respect to controllability expected in service.” That is, the thrust lossfrom the inoperative engine must be at the rate that would occur if an engine suddenly becameinoperative in service.
But you are correct that there is no official Vmca definition for the US FAA.