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I just bought a 1978 Kawasaki kz750 B runs great trying to register it in California the tags are back like 10 years and I can't find the VIN number so I can find a look up the last owner so I can change it over to my name.

It is on the steering neck. That is the front most part of the frame that the fork assembly is connected to, just behind the headlight. It is stamped into the metal and there should be a white sticker with the frame number (what you call the vin), and the date of manufacture. Keep in mind that the model year does not corespond to the calendar year. That is, new model production typically starts in August. To be absolutely sure of the model year, you can compare the frame number to the list on the back of the appropriate factory service manual, which should be your very next purchase.

(Edit: TexasKZ posted while I was typing...)The VIN number should be on the steering neck. Even with the VIN getting a title may be tricky. You will need a Bill of Sale and Statement of Fact from the guy you got the bike from describing where the bike has been all these years and why the title is lost. If the bike was previously registered in CA it will (most likely) have fallen off the record after 10 years unless it was Planned Non Op'ed, in which case the record will still be current. If it's PNO you will need a BoS and SoF along with signature from the registered owner. That can be tricky because the DMV won't tell you who the owner is.

Good luck!