VSTi support?

Does Dorico support VSTi instruments? There are some nice piano plugins that I have that are in VSTi format that I would like to use in Dorico.

Yes, of course, Dorico does support VSTinstruments. HALion Sonic SE (the default player in Dorico) itself is a VSTi.
If VSTis adhere to the VST3 standard, then they will work immediately with Dorico. VSTis that are built using the older VST2 standard, need to be whitelisted and (depending on the case) Dorico needs to be told the installation path. Both you do in the Dorico preferences dialog under the Play tab.