Technically VTTs can support placement, but, like most unique features, it’s dependent on the client player.  Vimeo supports it, YouTube does not, it's a choice.

It’s technically possible to add bolding to the code as HTML, but it’s very unlikely that the client player would be able to decode it. If the bolding didn't decode properly the words would either show up as plain text or the individual bolded words might not show up at all.  Most players don’t account for bold text since it’s not a standard in captioning specs anywhere in the industry.

Italics codes are much more likely to be readable, though again, there's no guarantee it'll ultimately work for the client's player of choice.