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If you have no non-compete and the non-compete is only between vendor A and employer A, this has no bearing on you leaving and they cannot stop you from doing so. The only one who could possibly be liable would be potentially Vendor A, since employer B is a client to Vendor A and Vendor A should have stopped employer B. However, that is not preventing you from leaving and unless employer B had some agreement with Vendor A, that is not their issue either. The only agreement is between Company A and Vendor A.