Following the issue reported in the last discussion, I found a tricky workaround that gets vLLM working with Solar-Open-100B for now.

Fix 1: ALLOWED_LAYER_TYPES ImportError

vllm/config/model.py, line 14:

# Before
from transformers.configuration_utils import ALLOWED_LAYER_TYPES

# After
from transformers.configuration_utils import ALLOWED_MLP_LAYER_TYPES
ALLOWED_LAYER_TYPES = ALLOWED_MLP_LAYER_TYPES

Fix 2: use_qk_norm AttributeError in SolarOpenDecoderLayer

vllm/model_executor/models/solar_open.py, in SolarOpenDecoderLayer class:

# Before
use_qk_norm=config.use_qk_norm,
# After
use_qk_norm=getattr(config, "use_qk_norm", False),

After these two patches, vllm serve upstage/Solar-Open-100B --tensor-parallel-size 4 loads and generates correctly.
These are hacky workarounds, not proper upstream fixes. But for anyone stuck right now, this should get you going.
Env: Upstage custom vLLM (0.12.1.dev1+solaropen) / transformers==5.0.0 / CUDA 12.8 / 4x GPU