When Wersi introduced the OAS instruments at Frankfurt, the Roland engineers were all over them, also Yamaha now own Steinberg, (The originators of the VST protocol) so out of the big 3, these 2 are the most likely candidates. Also since Steinberg started in 1984, there have always been close links with Wersi, (The first computer sequencer from Steinberg was identical to the Wersi computer sequencer launched in 1984) and up to OAS 6, Cubasis and Wavelab were included as standard on Wersi Instruments.
It is also no accident that Yamaha styles are now directly playable on a Wersi through the OAA (Open Art Arranger) software.
Either way, the future is open.

Bill