hard vang means no topping lift?
There are some exceptions. On my Islander 36, I had problems with the top batten hanging up on the topping lift after I switched to a full batten main. The roach on the new sail was greater than the old conventional batten main.
Each time I tacked, the batten would hang up on the topping lift. I had to keep the topping lift slack so I could flip it over the leach to allow the sail to take its normal shape.
The problem was solved by installing a rigid vang and removing the topping lift. It works very well. I do support the boom with the halyard when not sailing, but the vang holds the boom off the dodger just fine without it.