I don't understand this, does it mean to write more data into the block when there is more witness data? So.. increase the impact of "spam"? Or are miners currently not aware of the size of the witness data and can't compute the vbyte cost? Must be something else, because these two don't make sense?
The vbyte metric helps to adjust for the fact that witness data is discounted in SegWit transactions. For non-SegWit transactions (legacy transactions, not much used anymore), the transaction size in bytes is the same as the size in virtual bytes (vbytes). For SegWit transactions, the size in vbytes is typically less than the size in bytes due to the discount applied to witness data.
Miners prioritize transactions based on their fee rate per vbyte. Users may want to optimize their fees by considering the effective size of the transaction in vbytes rather than the raw size in bytes.
TLDR: Witness data is cheaper than the rest of the data in transactions. Bram proposes to make witness data more expensive. Making it more expensive may cause the users to pay slightly more on average than before Taproot but should help in pricing out spammers (an that should in turn drive down the fees).
P.S. Bitcoin's last soft fork, Taproot, is also called SegWit v.1. Not to be confused with SegWit v.0.