No problem, there's no rush. In the menatime, considering they are named similarly, are Coronas HD cache and Path tracing at all similar to Vrays Light cache and Path tracing? Would it help to watch some Vray tutorials on Light cache and path tracing to better understand Coronas settings/theory and what's going on behind them?
Not really, they work similar internally, but knobs are quite different. Considering PT and HD, here are all the basics you need to know:
Increasing PT samples increases amount of secondary rays per eye ray. secondary rays handle quality of GI, reflection, refraction (including glossiness of these parameters), translucency, and so on, while primary rays handle quality of AA, DoF and Motion Blur. So decreasing PT samples is suggested in scenes with fine geometry, that needs a lot of AA, or scenes with strong DoF and/or motion blur. While increasing PT samples is suggested in scenes with difficult indirect lighting. Default works fine in most cases, and i would never go under 2 and above 64 in PT samples parameter.
HDcache is simple, you can ignore most of the knobs, except two. PT samples in HDcache define qualit of HDcache samples, and precomp density their density. Default values work fine. Except for final renders, you can raise PT samples in HDcache to 512, or even 1024 if you have lot of complex indirect bouncing. And on the other side, for quick previews, you can drop precomp density from 1 to 0.2, and then put it back to 1 for finals :)
If you have windows, or some large glass elements like glass railings or glass walls, then set refraction mode of your glass to hybrid, so it will generate transparent shadows instead of caustics and render a lot faster. If you have glass planes in windows that are just one polygon, without thickness, then you can use twosided glass mode, which does the same, but generates no refraction. Similar to thin walled mode in Arch&Design material.
Lastly... watch diffuse brightness of your materials. Your should never have too bright (over RGB 220) materials. In mental ray, you may be used for this, as mental ray will give you contact shadows even on superbright materials because you usually set final gather to very low diffuse bounces, so light falls off a lot quicker than in reality. Corona has 25 bounces by default, so light does not fall of that quickly and you get more light bouncing = less contact shadows. Also, in Corona, superbright materials can slow down rendering a bit, unlike mental ray :)
