When exporting media from Premiere Pro CC (latest version as at 12 August 2018: ver 12.1.2 (build 69)), under captions, you give us the option to Create Sidecar File to .mcc and .xml and .srt BUT not to the most useful (for Vimeo/YouTube at least) .vtt ! PLEASE give us that option.
I would normally say that a possible workaround would be to export as .srt and then use Subtitle Edit and save as .vtt
But Subtitle Edit appears to save to .vtt only font color and not position or other information.
My tests were brief.
You can get an idea by testing, and comparing to the vtt spes: