Surely the VM is already update-friendly. If you update a tuple in a page with the visibility bit set, the bit must be unset or you will get wrong results.
I was referring in the context of index only scans to skip visibility checks. I doubt, whether the visibility map feature to skip visibility checks at the heap can be created without any extra cost to updates/inserts. When a data is compressed then there is more contention for the same block and hence would likely affect DMLs. I hope that's what Tom was also referring to, but not in the visibility map context.
