Donut on a factory unmodified 243 case? Seems unlikely to me.
As for fitting and feeding from magazine. Either seat deeply or get a wyatts mag box. You won't loose that much velocity with them deep seated. I'd give it a try.
If you do get the Wyatt's extended magazine you can still shoot factory ammo safely. Don't know if you will have any feeding issues from the longer magazine. Not sure if you realize the bolt stop will have to be altered so the bolt can be retracted further rearward. Also some metal will have to be removed from the front portion of the receiver's mag box recess to accommodate this longer mag box. Check out Wyatt's website to see the details: WYATT'S OUTDOOR, INC.
The issue with the VLDs is whether or not they will shoot accurately with a jump to the lands. If you cannot get them to shoot you could switch to the 105 match target hybrids which are not as sensitive to seating depth. I am jumping them .065" in my 6mm-284 and they shoot very accurately.
Not sure which VLDs you are using, hunting or target. If you are using the hunting version you may question using the hybrid target bullet for hunting. I shot a coues wt at 509 yds this year with the 6mm-284 and the 105 hybrid and it worked fine. Velocity impact just under 2800 fps. In fact it was far more devastating than the 115 VLDs that I shot several deer from a 257 weatherby at higher velocities.
Let us know how things turn out