Visionaire will automatically scale the game up/down to fit the resolution of the device it is being played on. The aspect ratio will remain the same, which means that black bars will added accordingly - actually these aren't black bars, but the background of the game window.
In regards to mobile devices, you have to take that into consideration when you develop your game as what might work on your pc might not work on a mobile device as a pc tends to be a lot more powerful. If you wanted to develop a 1080p game with lots of animations & rich details then it's very likely it wouldn't run very well on a mobile device/tablet because 1080p can be quite demanding & hdd costly. It might end up being better to work on a smaller resolution such as 1280x720 to reduce system requirements if you want to target multiple platforms besides pc or even create the mobile version in a completely different ved file.