it could be useful to share a sample of your logs.
Anyway, you could use something like this
| rex "Vlan819<VLAN>\w+)"
to test with a sample.
There are many ways. Here is another one:
... | rex field=VLAN mode=sed "s/\(.*$//"
| makeresults | eval VLAN="vlan(foo)" | rex field=VLAN mode=sed "s/\(.*$//"