Hey
QCE does have a lot of overlap with VCE but it certainly isn't a 100% match. In particular, topics like motion would match very very well. You could get lulled into a false sense of security by things like VCE using flux = AB (they only consider perpendicular and parallel scenarios whereas QCE is broader) rather than ABcos(theta) (which you need to use) etc. so you'd need to make sure you stay on top of knowing what is in the QCE syllabus and your equations.
As a quick summary:
Unit 3
Topic 1: QCE and VCE strongly align
Topic 2: Lots of overlap but also some points of difference - some subtle.
Unit 4
Topic 1: Most of this is the same as in VCE and covered there but there are some things QCE has that VCE doesn't (e.g. the paradoxes)
Topic 2: Very similar to VCE but, for example, black body radiation is in HSC but not VCE
Topic 3: Don't bother looking to VCE - you won't find anything useful in VCE units 3&4 for this
Note that there are also some things VCE has that QCE does not, e.g. light ray diagram stuff. In the above summary I have only focused on how well VCE does include what QCE has - you'd have to learn what topics to complete & which ones to stay away from. It's not so much that there are different difficulty levels but more that some dot points in the syllabus are different.
As suggested above I think it's a good idea to look at EdUnlimited so you can see for yourself