I had just started a LA job when I submitted my VMCAS. I only had 40 hours on my application, but mentioned in the description that I would be doing it for a 6 month, 500 hr commitment. If you haven't even started the internship yet, you can't put it on your application under vet experience. You CAN, however, put it in the "explanation" section. That's sort of the section where you can put anything.
I did something little unique and maybe a little crazy. I got a lot of different experiences after I turned in my VMCAS. So I drafted a letter indicating everything I had done since October, and sent it out to all the schools in early/mid-December. That's usually the time that they start evaluating applications (they use October and November to make sure all the requirements are met and they have all the materials...for the most part). I'm not sure if they even looked at that letter or evaluated it, but it didn't hurt me. I ended up getting into 4/5 schools that I interviewed at. A little obsessive and crazy, yes.
Basically, just do something to indicate your plans and commitments.