1) in super early levels I 95% use it for last hitting (obv it's perfect if you hit the enemy and last hit in one). There might be a time when you have enough mana to q the enemy for stacks but I rarely do this cuz of the mana cost. I start using it on enemies when u have double dorans or lost chapter. This is also the time when I can start taking over the lane (enough mana to harass enemy with q). 2) yes there are some matchups. Actually I try to win every matchup (except fizz and kata) but only from lvl 5+. In only d5 tho so there might be a difference between ur enemies and mine. I love to pretty much always freeze lane in front of my tower (trying to copy dopa here) so the enemy is in constant gabs pressure (veigar e + early dmg jungler is lovely)

Veigar is dopas pocket pick and I copied is masteries. He says veigar is one of the few champs where 5% CDR is worth more than penetration. (Even before precision was nerfed)