The MTU isn't set by vsftpd - it's done by the kernel network stack.
The behaviour you're seeing, however, is normal. From the MTU discovery RFC:
To detect increases in a path's PMTU, a host
periodically increases its assumed PMTU (and if it had stopped,
resumes setting the DF bit). This will almost always result in
datagrams being discarded and Datagram Too Big messages being
generated, because in most cases the PMTU of the path will not have
changed, so it should be done infrequently.