Click the PDC button (it turns blue) and see if that helps. With complex projects, the delay compensation can be enough that if you're monitoring yourself through Sonar, you'll hear an audible and horribly annoying delay. When you bypass PDC, you should hear yourself with no latency issues. The only complication is you have to re-enable it (gray) during playback.
I've only had to do this with projects where I'm doing something like a guitar overdub through an amp sim and there are tons of effects done on an object-oriented basis, as well as lots of other plug-ins.
I reviewed the 1818VSL with Sonar and it worked like a champ. Make sure that you have all the VSL control panel settings and such correct. IIRC when using it with a 16.0.2, it defaulted to something like 2048 samples and I had to set it properly. I'm sorry I can't be more detailed, but the system went back to PreSonus after the review was done.
It's probably a situation where you'll change one parameter, say "doh!", and carry on recording