The High Aldwin
Yes, he would. He didn't attack VECNA with his first attack/hit. He attacked the PILLAR next to them both.But Hjalman would have not gotten the Furry of Blows off, cause Vecna would have had two reactions.
That initiated his Attack action, allowing him to use Flurry of Blows, and the first of those hit Vecna, and he denied Vecna reactions. So, wthether he had one or two reactions, he couldn't use them.