VET = Vocational education and training; they are mostly run through TAFE and/or industry, covering trades (carpentry, automotive, building and construction) and things like hospitality, animal/vet stuff, cisco, community service etc. They can count as 5th/6th subject for your ATAR and (I think) the score is calculated as 10% of the average of your top four actual vce subject scores. A VET subject gives you (usually) a Cert II or Cert III qualification - good headstart into an industry while doing traditional academic work at school.
http://www.vcaa.vic.edu.au/vet/programs/index.htmlUni Enhancement subjects are like doing a single semester's worth of one subject at uni, there's a broad range available at both Monash and Melbourne (possibly some other providers, I don't know). I'm doing UMEP Maths (Melbourne course) and it's really interesting - I see you do humanities, so you could look into uni enhancement subjects in those areas - check out the Melbourne and Monash websites. This year, uni enhancement counts as a 6th subject, adding 4.0, 5.0 or 5.5 to your aggregate in ATAR calculation depending on how well you do, but I think the system is changing, so I'm not sure for next year.
I'd recommend a uni enhancement subject, the workload is less than a VCE subject and they are generally more interesting, taking your interests to a higher level. They can also count for credit towards a bachelor's degree you do in the future, if it's in a relevant subject area, and you could accelerate at uni to the second year subject in first year. It can also give you a taste of uni life if you do a course held on-campus. Melbourne's program is free, Monash charges something like $450 per semester. I'm not too sure.