Marc, yes, I was assuming VSP7200. HW and SW IST implementation are quite different between the two products. ERS switches are typically a bit more sensitive to short VLACP timers, especially when stacked. The VSP7000 was using the ERS operating system (BOSS), it is EOS since quite a while now.
yes, for IST deployments b) applies. I am not aware of any ASIC restrictions on the VSP7200 that would trigger not to use VLACP with IST/vIST. Keep in mind, if you are running the VOSS switch in non-SPB mode, the switch is still using vIST concept internally, even though you are configuring it as normal IST/SMLT, but this does not have any consequences for VLACP.