Evan Z wrote: Can I also use the same 2920 for MAD (LACP) detection on the VSF stack?
Sorry but MAD via BFD on IRF shouldn't require a third party device do be deployed (it should be deployed as direct and dedicated Switches inter-link). Isn't it?
I mean, since you have only a pair of HPE 5700 you - probably - don't need to save ports when implementing MAD BFD (you are going to daisy chaining them together with regard to BFD) and so you don't need to add an intermediate device (like when deploying MAD via LACP Port Trunks on VSF)...so you can just do MAD BFD directly between IRF Members.
If you want MAD BFD via an intermediate device it should be possible to use the same Aruba 2920 for both MAD BFD (to serve IRF) and MAD LLDP (to serve VSF).
Performing MAD LLDP instead requires that you use specifically a third party supported device like an Aruba 2920: in that case it requires only that you define a MAD dedicated VLAN Interface (with its VLAN Id and IP Address respectively equal and compatible with ones used on the VSF side) and configure a dedicated LACP Port Trunk (two ports should suffice: each one on Aruba 2920 should be uplinked to one port of each member of the VSF) to be tagged member of that MAD dedicated VLAN; this is the MAD mechanism configuration part (VSF MAD mechanism configuration part is somewhat specular plus the vsf command to activate the lldp-mad on IPv4 against the MAD dedicated VLAN Interface on the Aruba 2920 [*]).
Eventually, if you don't want to involve a third party MAD device specifically for VSF, you can use the OoBM way - as an alternative way to implement MAD method - between both 5400R zl2 Switches.
[*] See VSF Configuration Guide or relevant 5400R zl2 Manual's VSF dedicated Chapter.