Supposedly "Hard to root". "Hardest to root cultivar". Etc.
Obviously, in reading the edit on the last post, i thought I was having the same experience others have complained of, but I'm happy MBVS is easier to root than a lot of others. Just for laughs, when I got my first batch of Sal's (EL/Gene's), had 2/3 act as if they would root. The third, I potted anyway. The first two took off! The third....nothing. so, finally, after 6 weeks of nothing, I risked carefully removing from the pot, only to find zero roots! I put it back in the baggie, where it stayed for 2 months (!!!!) before it finally started roots. I pulled out, potted again, and the roots have exploded ;) Just goes to show you ... patience, patience, patience.