Hello Horst and welcome!
Yes, your two Aruba 5400R zl2 (deployed as a Distributed Trunking pair - a DT pair - with/without the additional VRRP enabled feature for IP Routing) can coexist together with a VSX Cluster on the same network and those two solutions (the DT pair on one side and the VSX Cluster on the other) can be physically interconnected together too, if necessary for your network design.
Let me say first that you could consider a VSX Cluster solution (moreover when it is deployed with the Active Gateway feature enabled) like a sort of DT+VRRP solution "on steroids" or, eventually, if no Active Gateway and VRRP features are used respectivey on each side, a VSX Cluster can be simply seen as a sort of new and developed DT pair (this in a side-by-side comparison at high level, when the focus is on what is seen by the peering devices connected to the DT pair of to the VSX Cluster).
If you look at both solutions from a pure Layer 2 point of view (so in a VSX Cluster versus DT pair comparison), then a VSX Cluster could be connected to a pair of Aruba 5400R zl2 chassis (configured to work on DT mode) by using VSX LAG(s) with LACP while, on the DT pair side, the corresponding Aruba pair should use the DT-Trunks (Distributed Trunking) in LACP mode (dt-lacp) approach: that way, each side will see the other - through its (Distributed Truning versus Multi-Chassis) aggregated links - as a single logical entity (in other words it is like having a DT pair with its uplinks to a single peer Switch or, vice-versa, like having a VSX Cluster with its uplinks to a single peer Switch). Each side (DT pair or VSX Cluster) will believe that the other is just a single logical entity...again...from the standpoint of each solution, the other will be seen like being a single logical entity (like a standalone switch is generally seen).
As example, have a read of this old thread:
and follow the cited link.
The fact that your current DT pair has/hasn't also an IP Routing role (like when VRRP is enabled/disabled on the DT switches pair) could/couldn't introduce additional design considerations if you plan to interconnect it with a VSX Cluster that has the Active Gateway role enabled (remember that a VSX Cluster + Active Gateway is like a sort of DT pair + VRRP "on steroids"...but the peer effect is that both solutions offer highly available IP Routing service and aggregated redundant connectivity to downstream devices)...these additional considerations will pop up simply because the solutions you're going to interconnected (at Layer 2), both have a Layer 3 role too (IP Routing) and, respectively, each one is acting as the router for its defined VLANs (SVI)...so your milage may vary depending what you're trying to achieve (migration of features? just simple Layer 2 networking extension while routing stay where it is today? etc.).