Cool! I made two games last year that targeted the original COSMAC VIP CHIP-8 implementation, and I'm working on another now. Historical accuracy is definitely one of the things I enjoy most in Octojam.
"Sprite drawing trigger vblank" means that when Octo encounters a `sprite` instruction, it will then halt and wait for the next frame is drawn before continuing execution, so drawing sprites will be a bottleneck for speed.
I think 7 or 15 cycles per frame is reasonable. It's all an approximation, of course, since in Octo all instructions take one "cycle" (AFAIK) while it varies between instructions on COSMAC VIP. Here's a table of real-world timing for each instruction.
The best approach is probably to test your game in the Emma02 emulator, which emulates the COSMAC VIP faithfully. As far as I remember it's a little hard to understand how to use it though.