Not a unimelb student, but I looked over the units anyway and it seems I've done the Monash equivalent, so my two cents:
Vector Calc - looks pretty easy, and is also a lot more relevant to an electrical engineering major. (both are, really, but this is moreso) Granted, I didn't have problems with vector calc, but know a few people who did - I'd imagine you'd find it slightly easier than most, though, given your pre-exposure in engineering maths.
Probability for statistics - normally, I'd say that an introductory probability unit is harder than an introductory vector calc unit, but this unit actually looks quite easy. Plus, I know the textbook being used, and it's a very straightforward resource probability-wise, and it doesn't look like you'll venture at all into the inference stuff. A little less relevant to your major, but I'd imagine that this unit would be easier.
I guess my final call would be this - if you're fairly confident with the multivariable calc you've done so far, and are comfortable with more of it, do vector calc. If on the other hand you're fine trying something new (probability at uni level is a bit different to the maths you're used to, unless you've done RA maybe?), go for probability. As I said, though - not a unimelb student, so someone who has taken both/either might have a different opinion.