No public utility of a class specified in the foregoing section shall henceforth exercise any right or privilege, or obtain a franchise, or permit, to exercise such right or privilege, from a municipality or county, without having first obtained from the commission a certificate that the public convenience and necessity require the exercise of such right and privilege: provided, that when the commission shall find, after hearing, that the public utility has heretofore begun actual construction work, and is prosecuting such work in good faith, uninterruptedly, and with reasonable diligence in proportion to the magnitude of the undertaking, under any franchise or permit heretofore granted, but not heretofore actually exercised, such public utility may proceed to the completion of such work and may after such completion exercise such right and privilege: provided further, that this section shall not be construed to validate any right or privilege now invalid or hereafter becoming invalid under any law of this state, nor impair any vested right in any franchise or permit heretofore granted.