For Juniper Networks EX4400 Series Switches, 25G ports cannot be used as virtual chassis ports. 

root> request virtual-chassis vc-port set pic-slot 2 port 3
error: PIC slot 2 is not valid for virtual chassis port

root> show version
fpc0:
-------------------------------------------------------------------------
Model: ex4400-48f
Junos: 22.3R1.11

root> show chassis hardware
Hardware inventory:
Item       Version Part number  Description
Chassis                 EX4400-48F
Pseudo CB 0
Routing Engine 0     BUILTIN    RE-EX4400-48F
FPC 0      REV 11  650-114385   EX4400-48F
 CPU       BUILTIN    FPC CPU
 PIC 0     REV 11  BUILTIN    36x 1G SFP, 12x 1G/10G SFP/SFP+
 PIC 1     REV 11  650-114385   2x100G QSFP28 VCP
 PIC 2     REV 04  650-107357   4x25G SFP28
  Xcvr 2    REV 01  740-071562   SFP28-25G-BASE-LR
  Xcvr 3    REV 01  740-071562   SFP28-25G-BASE-LR

25G SFPs are getting error messages when attempting to configure VC-Ports

The 2x100G/40G interfaces or the optional 4x25G extension module interfaces can be configured as Virtual Chassis interfaces or network interfaces without rebooting the switch again. Use the request virtual-chassis vc-port set pic-slot 1 port <port> command to convert the interface to a Virtual Chassis port and the request virtual-chassis vc-port delete pic-slot 1 port <port> command to convert the interface to a network port.

You can also also convert 4x25G network ports of the uplink module in PIC-slot 2 as Virtual Chassis ports. To convert the 4X25G network ports to Virtual ports and vice versa, use the request virtual-chassis vc-port set pic-slot 2 port <port> and request virtual-chassis vc-port delete pic-slot 2 port <port> commands.

But even trying the command explained above, the following error will appear:

root> request virtual-chassis vc-port set pic-slot 2 port 3

error: PIC slot 2 is not valid for virtual chassis port

• To accomplish the VC port conversion in PICK 2 for 4x25G network ports, run the below commands, this will reboot the switch.

{master:0}

root@c04-31> request virtual-chassis mode hgoe reboot

fpc0:

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

Mode set to 'Virtual Chassis with hgoe mode enabled'. Rebooting system...

• Once the device is up, you may be able to convert the PIC 2 as VCP:

root@c04-31:RE:0% cli

{master:0}

root@c04-31> request virtual-chassis vc-port set pic-slot 2 port 2

Port conversion initiated, use show virtual-chassis vc-port to verify

{master:0}

root@c04-31> request virtual-chassis vc-port set pic-slot 2 port 3

Port conversion initiated, use show virtual-chassis vc-port to verify

• This information is displayed here:

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.

EX4400 Switches in a Virtual Chassis

EX4400 Switches in a Virtual Chassis