How to schedule backup job for most recent data, without having full backup?

A NAS device has VM machine backup set's which is from another backup software. The NAS is listed under User defined selection with Backup Exec.
I would like to schedule a backup only for the most recent backup set, so that the most recent backup sets can be stored on a tape.
In order to achieve that full backup and followed by incremental backup would work, but I do not have sufficient storage to take full backup of entire NAS and followed by incremental backup's.
How do I achieve this?