if you have a vlan.dat file it is because you have an etherswitch module installed in the router.
You can upload the file using
copy flash tftp:
to a PC running TFTP server for later download to the new flash using copy tftp: flash command
Or if you have another network device like a LAN switch that speaks VTP with the etherswitch module on a trunk port, you can make it a VTP server so that it will save a vlan.dat into its flash. Later the etherswitch will be able to retrieve the vlan.dat file from the external LAN switch.
You can check with dir /all all the filesystems that are on the router, the etherswitch module might have an internal bootflash in that case it might be used to copy the vlan.dat and to retrieve it later.
Hope to help