The sandbag is pressing against the rifle, but isn't the bipod bolted directly to the stock, which is kind of the equivalent of having contact with the whole thing?
Also, is the vibration from the bipod happening before or after the bullet leaves the barrel? Is it going to affect the point of impact or just how fast you can get back on target?
As long as the barrel is truely free floated it doesnt matter wether you rest your rifle on the bipod or the stock itself. I think you might be confused on how to use the sandbag, you place the sandbag under the rifle stock, not forward of the bipod touching the barrel. Also the flatter you can get the sandbag the better, because you can get more sandbag to come into contact with the stock.
(You can also place a small sandbag which people call shooting bags under your actual butt stock to further stabilize your gun)
The vibration from the bipod is minimal to none at most times. But if you're shooting on a concrete slab or a bench with multiple shooters, its going to pick up some vibrations. A sandbag will act more as a shock absorber and really take any movement out of the equation. The point I'm trying to make about a bipod is your suspending a rifle in the air off two small legs. Which is sturdier, two small legs holding up the forward end of your rifle or a large sandbag that forms to your stock? Arguably the latter.
When it comes to changing your point of impact, it shouldn't change at all.
BUT if your barrel isnt free floated, you could get a different POI between a bipod and a sandbag.
With all that said, its up to you and your shooting technique to wether or not you'll get back on target faster with either a bipod or sandbag. Speed should be your last concern though considering you're doing load development. It should all be about consistency and taking your time. Knowing the kind of rifle your using, the rifle stock your using, the bipod your using, the sandbags you have access to, and the bench you'll be shooting off would help people give advice.
Personally, if I was trying to get as accurate as possible, I wouldn't be shooting off a bench. I'd be shooting prone, using either a high quality bipod on actual earth OR sandbag under the stock, and another small sandbag under my buttstock to change elevation.