ah! parse the vtt yourself (regex!), display the tiles at the respective time intervals assuming all the info is in the vtt file.
Different types of implementations sample here: https://github.com/rokudev/samples/tree/master/certification/trickplay-samples-master
Thanks for the response
Can you confirm that Roku will accept a single thumbnail tile displayed when using Trick Play?
All the examples I have currently seen have multiple thumbnails.