If you are using VuFind 3 or newer, you can use the SOLR_PORT environment variable to control this. For example:
SOLR_PORT=8081 ./solr.sh start
Then you just need to adjust the url setting in the [Index] section of config.ini to point to the expected port.
This wiki page contains more details, including instructions on how to do this in older versions of VuFind:
I hope this is helpful!