diff --git a/src/axolotl/core/trainers/grpo/__init__.py b/src/axolotl/core/trainers/grpo/__init__.py
index 839c20c2e..4106a2a7d 100644
--- a/src/axolotl/core/trainers/grpo/__init__.py
+++ b/src/axolotl/core/trainers/grpo/__init__.py
@@ -49,7 +49,8 @@ class GRPOStrategy:
 
         if trl.use_vllm:
             grpo_args_kwargs["use_vllm"] = trl.use_vllm
-            grpo_args_kwargs["vllm_mode"] = trl.vllm_mode
+            if trl.vllm_mode:
+                grpo_args_kwargs["vllm_mode"] = trl.vllm_mode
             if trl.vllm_mode == "colocate":
                 grpo_args_kwargs["vllm_gpu_memory_utilization"] = (
                     vllm_cfg.gpu_memory_utilization
diff --git a/src/axolotl/utils/schemas/validation.py b/src/axolotl/utils/schemas/validation.py
index 502c18e7d..aa249c6ce 100644
--- a/src/axolotl/utils/schemas/validation.py
+++ b/src/axolotl/utils/schemas/validation.py
@@ -1268,6 +1268,19 @@ class DistributedValidationMixin:
         return self
 
 
+class GRPOVllmValidationMixin:
+    """Validation mixin for vllm when using GRPO."""
+
+    @model_validator(mode="after")
+    def check_vllm_mode_set(self):
+        if self.trl and self.trl.use_vllm and not self.trl.vllm_mode:
+            LOG.warning(
+                "vllm_mode must be set to either `server` or `colocate` when using vllm, using default value `server`"
+            )
+            self.trl.vllm_mode = "server"
+        return self
+
+
 # pylint: disable=too-many-ancestors
 class ValidationMixin(
     DatasetValidationMixin,
@@ -1281,5 +1294,6 @@ class ValidationMixin(
     PretrainingValidationMixin,
     ModelCompatibilityValidationMixin,
     ComplexValidationMixin,
+    GRPOVllmValidationMixin,
 ):
     """Full validation mixin for Axolotl configuration."""