How was that engine running before you did the head?

Do a compression / leak down test, you will know right away if something is
wrong with the valves. But i doubt that, you didn't talk about noise or
heavy engine shake.

Also, check the plug to see if one or 2 is wet (coolant or fuel). Plug tell
everything.

If not, you broke or didn't plug something right in the head job process.

In my experience, if a pushrod is not well align, it will pretty quickly
create an horrible noise.... i said horrible.

Now, if none of the above, find if you have a fuel pressure probleme or a
ignition (fire) one.

Last, and sorry to say, Vanagon head job is an art in some ways. I have fix
badly done job more than once. I can change Subi / Honda / Toyota head
gasket with my eyes close.... Any Vanagon heads job still need my full
concentration.

Best luck to you
Ben