Convert random variables to value variables (#7284)

pymc/sampling/mcmc.py

@@ -742,6 +742,7 @@ def sample(
 
     if var_names is not None:
         trace_vars = [v for v in model.unobserved_RVs if v.name in var_names]
+        trace_vars = model.replace_rvs_by_values(trace_vars)
         assert len(trace_vars) == len(var_names), "Not all var_names were found in the model"
     else:
         trace_vars = None