When trying to copy something, like software, from a FTP-server, you are not able to specify source IP or interface which may be troublesome.
You can however configure your router to use a specific IP/VLAN as source.

cisco# configure terminal
cisco(config)# ip ftp username myusername
cisco(config)# ip ftp password mypassword
cisco(config)# ip ftp source-interface vlan 1234