Try updating all the plugins to their latest versions.
I just checked with VS Core, VS Battle, VS Party, and MK_Summon, and they work fine for me.
When my plugin works with CGMZ but not with VS, it is possible that the problem lies more in VS's hands than in mine.
Temporarily bypassing the party limit (i.e. 5 actors when the limit is 4) is not well supported in RPG Maker. Yes, there is the "exceed limit?" option in MK_Summon, but I no longer work on it. Also see its description on my blog:
https://decinbr.itch.io/rpg-maker-mz-plugin-party-swap-css-style
This plug in works with your MK summon and it has a party scene like visu stella. I would recommend this to others having the same problem as myself. It p robably just saved my game. I can now use both plug ins and return to my work. Thanks.
The problem for me is that visu stella party plug in wants the battle actors to be limited to 4. It will not allow me to have 5 actors in battle. I dont know why but it will not. And as soon as I disable it I can summon and I have 5 actors in battle LOL. So unfortunately I had to let it go . Thankfully theres other party plug ins