At common law, theft occurred when someone took the property of another with the intent to permanently deprive them of the property. There are a number of theft statutes in California that essentially make it crime to take or keep property that does not belong to you.
If you know where your property is you should demand it, and if they refuse to return it to you, contact the police. Without more facts, it is not clear whether there has been a theft crime or not, because, you gave them permission to take the property. Then you gave them permission to keep the property. That said, once you ask for it, they ought to return in. Of course depending on the circumstances surrounding you refusing to accept the property, they may have disposed of the property in a manner that will not constitute any theft, but may put you in a position to recover in a civil action.
You should consult with an attorney regarding your recovery options. This response is provided for academic purposes only.
I think your question is about civil liability, not criminal liability.
In this case, from your description, it sounds like you might want to sue this person for what we lawyers call "conversion." Conversion is the "wrongful exercise of dominion over the property of another."
If you plan to file suit anywhere other than small claims, you should hire an attorney.
This is a civil wrong, and therefore conversion, which is pretty much "civil theft" involving the wrongful withholding,destruction, or interference with personal property that belongs to someone else. Your claim is based on a breach of the private business contract you had with this vendor, and you've already filed a civil suit based on that breach. Because you refused the goods, you have remedies according to the CA Commercial Code.
The police, too, will consider this a civil matter between private parties and would not consider it criminal theft. The DA prosecutes theft and other crimes on behalf of the state and and its citizens when wrongs against the community are involved.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.