Update on this.  After way too much testing we realized the Junos version shipped on the QFXs did not support VC (even though the units DID for a virtual chassis!!!).  So we updated to 19.3R2-S3 and that fixed the issue.  Information wasn't immediately obvious on the Juniper support site.
I have a pair of S4810s configured via VLT, they are my core, running OSPF.  I have 5 Juniper Access VCs LAG'd via VLT peer lag settings to the pair with stretched L2 and a L3 management IP.  Works great.  I have now added a Juniper QFX 5120 VC LAG'd to the S4810s via two of the 40G ports.  Bundle forms and L2/L3 works as expected.  LLDP shows neighbors, no problems. What I want to do is swing the Juniper Access VC LAGs over from the S4810s to the QFX VC.  When I do this, I get very odd results. I can ping the moved Juniper Access VC from the QFX.  I can also often ping it from one of the S4810s, but not both.  But I'm no longer able to ping the moved VC from other segments.  I'm aware that during a fault scenario, VLT will not pass ICMP but the disruption impacts more than just ICMP.  If the same management segment is tagged to the Po on the Dells and the ae on the Juniper VC, I should be able to ping the access VC regardless of the Po/ae it is connected to. We are using OSPF to advertise routes.  show vlt mismatch indicates no errors. RSTP does not appear to be blocking. VLT Config (for one of the two S4810s) for example : vlt domain 999peer-link port-channel 128back-up destination 10.31.10.61primary-priority 1system-mac mac-address a0:10:10:00:03:e7unit-id 0peer-routingpeer-routing-timeout 10 interface fortyGigE 0/52description VLTino ip addressmtu 9252no shutdown interface fortyGigE 0/60description VLTino ip addressmtu 9252no shutdown interface Port-channel 128description PCH-128-VLTino ip addressmtu 12000channel-member fortyGigE 0/52,60no shutdown #show vlt statVLT Domain Statistics-----------------------HeartBeat Messages Sent: 106881849HeartBeat Messages Received: 106897332ICL Hello's Sent: 35627280ICL Hello's Received: 35632444Domain Mismatch Errors: 0Version Mismatch Errors: 0Config Mismatch Errors: 0 VLT MAC Statistics--------------------L2 Info Pkts sent:77828, L2 Mac-sync Pkts Sent:2508527L2 Info Pkts Rcvd:116139, L2 Mac-sync Pkts Rcvd:2633496L2 Reg Request sent:1L2 Reg Request rcvd:2 L2 Reg Response sent:1L2 Reg Response rcvd:1 VLT Igmp-Snooping Not Enabled VLT ARP Statistics--------------------ARP Tunnel Pkts sent:17867057ARP Tunnel Pkts Rcvd:130235814ARP Tunnel Pkts sent Non Vlt:2893675ARP Tunnel Pkts Rcvd Non Vlt:26952255ARP-sync Pkts Sent:2453484ARP-sync Pkts Rcvd:1120365ARP Reg Request sent:2ARP Reg Request rcvd:2 VLT NDP Statistics--------------------NDP NA VLT Tunnel Pkts sent:0NDP NA VLT Tunnel Pkts Rcvd:0NDP NA Non-VLT Tunnel Pkts sent:0NDP NA Non-VLT Tunnel Pkts Rcvd:0Ndp-sync Pkts Sent:0Ndp-sync Pkts Rcvd:1Ndp Reg Request sent:1Ndp Reg Request rcvd:2 VLT multicast not enabled #show vlt briefVLT Domain Brief------------------Domain ID: 999Role: PrimaryRole Priority: 1ICL Link Status: UpHeartBeat Status: UpVLT Peer Status: UpLocal Unit Id: 0Version: 6(7)Local System MAC address: f4:8e:38:04:7f:05Remote System MAC address: f4:8e:38:03:92:0bConfigured System MAC address: a0:10:10:00:03:e7Remote system version: 6(7)Delay-Restore timer: 90 secondsDelay-Restore Abort Threshold: 60 secondsPeer-Routing : EnabledPeer-Routing-Timeout timer: 10 secondsMulticast peer-routing timeout: 150 seconds Has anyone seen anything like this?  I've played with making the Po that links to the QFX hybrid, but that has no impact on what is occurring.
Swapping LAG from VLT to Juniper VC