Yeah I think really need to recreate the DTM in Vectorworks to perform all the site modifying tools... So far how I do it if I have a model from ArchiCAD, I saved it as .vrml format, extract all the coordinates, save it as .csv format, and then make it as a survey input data in Vectorworks to recreate the site model... Unfortunately yeah the IFC format so far served mostly just as 'reference' in a way..
By the way, is there anyway to export the DTM from Vectorworks to get just the coordinates/survey data?