A simple method is to just create a virtual-tag using the following:
$If($Contains(<Bitrate>,VBR)="T",,$If($RxMatch(<Bitrate>,"\d+")<320,<Bitrate>,))
And name it something like "WeakBits", then do a <Ctrl+F> search matching <Weakbits> "
has a value".
There may be other ways too, but this one works for me. Note that it'll only really work for MP3's, since .FLAC's don't have VBR as such, but can obviously render at all sorts of bitrate values.