I also think that it's vBytes, the fee rate is also based from vBytes not Bytes.
Because if the minRelayTxFee is based from Bytes, then 1input-2output 1sat/vB TXns w/ SegWit data will be lower than the
default minRelayTxFee & FeeFilter which would be rejected by most of the nodes, which doesn't happen.

Example:
A SegWit transaction of 166vBytes with 1sat/B transaction fee would have 0.67sat/B if it will be computed based on Raw Bytes (approx 247B).
Or just take a look at blockstream's data from a random SegWit Tx: 5de78791f4c7d545c5c03eda1953a62da7d4b9a07416687c851988b57ce1807f
"Transaction fees" is based from vBytes.