Samsung has held back 2.3 from the Nexus One to help sales of the Nexus S
Samsung is holding back 2.2 from the Vibrant to help with sales of the Vibrant Plus
Samsung now going to hold back 2.4 from the Nexus S to help sales of the Nexus S Plus?
History indicates yes
1) Google is the one that releases OTA updates on that phone and
2) Nexus One wasn't built by Samsung. HTC manufactured it.
Samsung is not going to hold back 2.4 from the Nexus S to help Nexus S Plus. Google is the one that controls the OTA updates... not Samsung. If anything, it would be Google holding back the update.
As for the Vibrant for the sale of the Vibrant Plus, that's all speculation.
I don't see how updating the original vibrant to 2.2 will cause anyone to pick the original vibrant over the vibrant plus. If they both have the same android OS version, the vibrant plus will still the better phone spec wise. There's no competition between the two since the vibrant plus is superior in every way.