[FIX] guidance: use max(vocab_size, len(tokenizer)) for n_vocab (#33509)

Signed-off-by: Frederic Odermatt <frederic.odermatt@44ai.ch>

vllm/v1/structured_output/backend_guidance.py

@@ -93,7 +93,7 @@ class GuidanceBackend(StructuredOutputBackend):
         )
 
         self.ll_tokenizer = llguidance_hf.from_tokenizer(
-            self.tokenizer, self.vocab_size
+            self.tokenizer, max(self.vocab_size, len(self.tokenizer))
         )
 
     def compile_grammar(