Attempting to run vc-ports at 10g may end up showing them UP but they will be unable to negotiate LLDP with the peering member, thus, no neighbors showing.
 

root> show virtual-chassis vc-port

fpc0:

--------------------------------------------------------------------------

Interface   Type         Trunk   Status    Speed     Neighbor

or                        ID              (mbps)    ID Interface

PIC / PORT

2/0         Configured    -1     Up        10000 

2/3         Configured    -1     Up        10000
 

“NOTE: 

In HGoE mode, on rear-panel ports (PIC slot 1) only 100G and 40G are supported as Virtual Chassis ports. On the front panel, 4x25G uplink module ports (PIC slot2) 25G ports are supported as Virtual Chassis Ports. 10G speed/10-Gbps uplink module ports are not supported as Virtual Chassis ports.”

https://www.juniper.net/documentation/us/en/software/junos/virtual-chassis-qfx/topics/concept/virtual-chassis-ex4200-overview.html

There is no workaround to enable 10G VC-ports on the EX4400 switches. For now, vc-port is supported only at 40g/100g on PIC 1 and at 25g on PIC2.