First of all, popular VMSes do not want to integrate with you. I am confident saying this, even without knowing who your company is.
A popular VMS wanting to integrate cameras is like a swimsuit model looking for a date. Do not hold your breadth.
It's a huge pain for VMSes to integrate with cameras, given the vast number of APIs, how often the APIs break, etc. Worse, they already support hundreds of different models at a very significant ongoing cost. So VMSes typically do not want to add any additional work for new brands.
Whatever negativity you have heard about ONVIF, ignore it. It is, bar none, the best way to get integration with a VMS.
(1) It may work out of the box but even if it doesn't:
(2) It's a lot easier for either you or the VMS to figure out what small parts of the integration do not and then to fix / modify just those (far better than starting from scratch).
Since pretty much all the popular VMSes support ONVIF, this is clearly your best choice.
If you really want direct integration, be prepared to either bring a very big deal (hundreds but more likely thousands of camera licenses that will refuse to use that VMS unless they go with your cameras) or be ready to pay tens of thousands of dollars.
Btw, VMS popularity varies by region, more so than cameras, since VMS tends to require more localized support and translation for individual languages.
WOW, as usual, super helpful John. I'm new to the industry so please bare with me. I'm simply trying to understand the surveillance market a bit better since I was recently hired as the marketing guy for a particular CCTV brand. I'm not interested in marketing it on your site, only to truly understand what the market and customers need to improve the products we have. It's all new to me, but I'm learning as I go here.