I didn't go to Vetsim, but I did go to VML in 2013...I think VML, at least, was definitely worth it, since I've made friends through it that I still talk to and I felt a lot more prepared to go into vet after attending (ie. knowing to avoid the debt trap that is St Georges Grenada!) However, what I gathered from that was that you had to do VML to do Vetsim? I'm not sure if that's changed.
In other words, Vetsim is another way for the company to make money out of you, and, while it is useful, you've got to decide whether you'd rather use the money to go on a more expensive/farther away work experience placement, as this will be worth more to you in the long run (since mentioning Vetsim on your PS will do nothing at all, but an individual and interesting placement might).
I hope that helped!