U21 and NT games are supposed to provide additional experience -- and 1 NT/U21 game provides more experience than regular league games. In general, however, the exact rate of experience gain for either is not know.
My personal observation is that 1 level of experience takes as much as 2 seasons for players who start regularly on my team, and that NT games do not seem to provide the gigantic experience amounts that they do in Hattrick, for example. The only think I can suggest is patience
As for Vukovski, we're grateful for your good work with him. Please keep it up, as I can use him in the senior squad when the time comes
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."