Difficult to say why you are getting drop outs. There are so many parameters that influence the performance. Starting with the spec of your machine, going over the audio interface and the used driver, the complexity of the project, the number of instruments used until the individual plug-in settings. VST2 doesn’t mean necessarily bad performance, it depends on how good the programmers did their job.
Furthermore, have a look at UVI’s own settings. E.g. HALion Sonic SE has an options page where you can control how many cores it will use and the maximum number of voices. Playing with these parameters can already improve the drop out situation.