Concerning the MX single chassis question, you need to open a ticket with Dell to validate if this is possible or not.
As for the question regarding deploying a single instance of VCF on VxRail and MX, then that is not possible. VCF on VxRail uses a specific deployment methodology where VxRail Manager integrates with the VCF SDDC Manager that are designed specifically for VxRail. MX Blades are treated as standard vSAN Ready nodes and hence you can use the vanilla VCF deployment methodology. Hence you have to end up with 2x VCF instances.