Funny story once my horse's neck was so sore after vaccinations that he couldn't walk straight for 3 days. I still rode him at the walk and trot on a loose rein to try to regain his range of motion faster tho.
Vaccinations work by injecting a small amount of the disease so that the horses can develop the antibodies to fight it off. This means they might be a bit sick for a few days after, so they need rest because a lot of their energy will be going to fighting off the disease
That's better not to rid him, at least not working during the 2 or 3 first days after the vaccin, or complet rest the first day and soft work the following days. Many horses are like us, tired and as Tessy said, a bit sick and that's better for them to have some rest :)