Recently I updated Vessel to work with the testnet as well. Its quite nice stuff and has made my testing with the testnet a bit easier. It's pretty important to make sure what we've got work with future forks, and the testnet is a great way to do so. Nothing affecting vessel was changed so as long as you are on v0.5.0 you should be fine and not need to update, but if you'd like to checkout the testnet, you get the latest version from here: https://gitlab.syncad.com/hive/vessel/-/releases.
To get onto the testnet with vessel, all you have to do is supply it with a proper testnet node. Right now we have https://testnet.openhive.network. You can set that over on the settings page on vessel, or when first importing your account. With this instance, most accounts should already be exported with the same keys so you can just use it with no problems.
Switching to the testnet also gets your vessel to change its color to a lovely pink and some of the pages will show the testnet symbols instead of the mainnet one to signify that you are on it.
I hope that this makes things easier for those of you use participate on the testnet.
I also did upgrade someguy's hivefeed-js(https://github.com/someguy123/hivefeed-js) to support the testnet as well. Just set the network to testnet in the config and you'll be good to go. Having more of our apps supporting the testnet will be awesome to make testing even easier and to make sure they function without problems once the fork comes around.
Nerdier part starts here
The main change that needed to be done was just changing Hive and HBD to the proper testnet symbols(though the latest hive-js version does this automatically so if I waited could have done less work haha), that can be seen on this commit https://gitlab.syncad.com/hive/vessel/-/commit/36b366d375cffb371f04a56b97a81de9d43d4d38. It was just a simple replacement of HIVE to TESTS and HBD to TBD.
The other part was to set the chainid and prefix to the correct values, which can be seen here: https://gitlab.syncad.com/hive/vessel/-/commit/67a395539180fba62f4c849418982706e5a07c2f. 
