Since the vegan/plant based lifestyle is still pretty new in terms of market and education, its still a rather free playing field in terms of education. Most 'certified' Chefs (that have already attended culinary school) are now starting to integrate into veganism. If you want to be at the 'head of the class' this is probably the long but best way; and probably the easiest way to move into Management and Top Chef based positions. This would involve going through a program to cook all dishes including non-vegan food. This might pose a problem as I was told I may have to 'taste' some non-vegan Foods during the program.
However, you can also become a skilled plant based vegan chef with strictly experience in the industry. There are many plant based schools that will offer a certificate, however it will not certify you as a chef. 'Chef' is actually a term for skill and not education. I have listed my personal list of top schools below. (Please keep in mind that some of these schools are not based in the US.)
and many others....
And before you decide to go through a program, make sure the credential is accepted by the organization to which you choose to apply. If you plan on working for yourself though, it might not matter as much.
Hope this helps.