I getting mixed responses. Mostly negative, but still mixed. One tax-brousse operator thought that there were 4x4 taxi-brousses connecting Vangaindrano to Taolagnaro. Another suggests you need to swim. Somewhat dated online sources seem to back up the second guy.
The second operator also suggested that it was possible to there in a taxi-brousse via Ihosy and RN13. Has anyone tried this route?
I don't really have the budget for two flights or a private 4x4.
You certainly don't need to swim - I drove from Manakara to Taolagnaro via Vangaindrano in August 2009. There are plenty of rivers, but they all have ferries.
The road is good quality blacktop till a few kilometres short of Vangaindrano, but south of the town it's only passable for 4x4. From what I could gather, at that time there was either no taxi brousse or only a very irregular service. Local people ask vehicles leaving Vangaindrano for a ride - you could try that yourself. There are a fair few vehicles travelling south, and I got the impression that most drivers were willing. Offer to contribute to the fuel costs.
I can't answer your question on the RN13 from Ihosy, but there are definitely many taxis brousses taking the RN10, which leaves the RN7 at Andranovory and takes you down to Taolagnaro via Betioka and Ambovombe. Nice route, but a long way round compared with the coast road.