I don't think it's confirmed it's Vegeta's assist being nerfed- could be his DP? But I don't think that's really all that bad if not beneficial. Nappa is on the buff list for starters so it's safe to assume they have a chance to make the character not so goddam assist dependent.
And Vegeta's assist even with the blockstun duration reduced from the current 1000000000e+10 frames still has a ton going for it:
1. good aggressive screen angle. Arguably better than beam assists in certain cases
2. holds the opponent in place, so still ridonkulously easy to combo off
3. lower risk of happy birthdays due to Vegeta's position in the air during the assist