Am I entitled to my full vesting on both 401(k)s now that I am cumulatively employed for over 5 years?
Most would agree that you are "entitled" to the money but there is nothing forcing your employer to honor that. The two companies are still distinct regardless of who owns them, but what you describe is certainly possible and has been done previously. If both 401(k)'s are at the same broker then it should be even easier for them to simply give you the full vesting in Company A's 401(k) and roll it over to Company C's.
A few years in, I transferred to company C
If your move was more of an in-company transfer instead of a "retire from A, hired by C" situation, then you should make sure to mention it. But again, it's up to your employer. If you have a contact with Company B that may also help.
See also the answers to this similar question (although it applies to someone at the beginning of the transfer process). If they won't give you the full vesting you can request some other type of compensation. Since you have already been working at Company C for a while they might not agree to it, but it doesn't hurt to ask.