Is Vivid Strike a reboot of the Nonoha franchise?

Nobody knows quite what the heck Vivid Strike is, but it's definitely not a reboot. That would imply gong back to Nanoha and Fate's story and telling it in a new way. But in Vivid Strike, neither of those characters even seems to exist.
The show is very coy about where, if anything, it falls in the continuity. Characters make offhand remarks, particularly about Einhart's past, that could be seen as referring to the events of Mahou Shoujo Lyrical Nanoha Vivid. But it's always throwaway comments with no effect on the story.
The premise could certainly has similarities to Vivid (Vivio and friends training for martial arts tournaments) but a lot of the details are different and the role of magic in particular has been heavily downplayed. It's definitely not a Vivid sequel either.
At the moment it can be taken as entirely its own thing, borrowing heavily from the Nanoha concept and particularly from Vivid, but showing no intention of acting as a prequel or sequel or whatever.
Technically, Vivid Strike is sort of a spinoff of Mahou Shoujo Lyrical Nanoha Vivid, which is a spinoff of Mahou Shoujo Lyrical Nanoha, which is very very loosely a spinoff of an appearance by Nanoha in a Triangle Hearts 3 drama CD, which was a side story/alternate take on the character from the Triangle Hearts 3 visual novel.
Yes, the Nanoha franchise has the strangest origins of any series ever.