As an alternative if you go to the form you are interested in via the Design Studio, then do Form Actions --> Embed Code
, you will get some code that looks like the following:
<script src="//app-sj04.marketo.com/js/forms2/js/forms2.js"></script><form id="mktoForm_14"></form><script>MktoForms2.loadForm("//app-sj04.marketo.com", "999-XXX-999", 1234);</script> Whatever value you have where 1234 is located is the form id (formid). I believe this is the same thing as a vid which is also reference in the documentation.