Don't even bother getting sightstone until you got your core items. Usually, its past 20 minutes when you need vision. In soloq, there are so many jungler that just don't know how to gank properly, or just don't gank at all. In 90% of games you can be safe just by using your trinkets. As mage support, you would rather have a bunch of cdr and mana/manaregen, so you can effectively poke enemies and have some extra mana in case of fight/gank. Winning lane almost equals to winning game right now. If you managed to get ahead of your opponent as a mage support, you better invest it in either Burst or Anti-Burst(Locket). And only then you can go for vision. Usually it happens by 20 minute mark, and there is not much point investing into vision, it's not gonna hurt unless they are playing as a team, but again, it's soloQ, so i would not count on that.