Reaper for Windows comes in two versions – 32 bit and 64 bit. The 64-bit version is an obvious choice for those of us who run the 64-bit version of Windows 7. That combo lets you use all the memory installed in your computer, instead of just 4 GB which is the limit in the 32-bit version.
There is one drawback though, since VTSs are compiled binary DLLs, they need to be in the same format as the VST host (Reaper in this case). Most commercial VSTs come in both versions, but a lot of freeware and older VSTs are available in 32-bit versions.
To remedy this Reaper has a VST bridge, a helper program that loads 32-bit VSTs in 64-bit Reaper. For a long time this bridge has loaded all 32-bit VSTs in a separate Window instead of inlining them in the FX window for a track. This is quite annoying, but fortunately there is now a solution.
When adding a VST, just right-click on it and choose Run as -> Embed bridged UI. With this option checked, all VSTs will load integrated into the FX window.