I am not sure if this should be something that each sampler should implement on its own, or if the VSTi API should implement it in a standarized way, but in any case, here goes:
HALion 3 has this RAMsave function. Not sure how it works, but I think in any sampler or sample-based VSTi, there should be the option to scan the MIDI files that are playing through the VSTi. The sampler then keeps in RAM only those samples that are needed to play the file. You can set an option so that this happens automatically upon opening a project. That would make projects load so much faster. And of course, if you are editing the MIDI files, or if you are playing live, then you could disable this function (i.e. load all samples to RAM).