Can someone who gets engaged during the 3 weeks buy a dress for her vort? (assuming for the sake of this question that the vort is during the 3 weeks but before R”C Av)
The issue during the three weeks is not buying the dress, but wearing it, because we make the shehechiyanu when wearing it. If the dress is not an expensive one, and wouldn’t require a shehechiyanu then it isn’t an issue, additionally there are people that don’t say shehechiyanu on new clothing, for them it is also not an issue. However, if it is your minhag and a dress that would require a shehechiyanu, then it is better to buy it and wear it a little bit before the three weeks. If not then wear it on Shabbos, (not during the nine days), and make the shehechiyanu then, if not and you have no choice then you can wear it. Regarding making a shehechiyanu on it, (and also on the jewelry that she will receive) if it her minhag to make shehechiyanu she can.
O:CH 551- 6, 17 Magen Avrohom 551-21, M:B 551-45, Shar Hatzion 551-48, Igros Moshe O:CH 3-80, Aruch Hashulchan 551-38. Some poskim say that since she needs the dress specifically for the vort, and the same with the a jewelry, that it is similar to a fruit that can only be used now and that the bracha is said. Also see Kovetz Halachos (Bein Hametzorim) 5-9, Kol Hamisablim Oleha pg. 189.