That depends what you mean by "low". The F-35B (like any fixed-wing aircraft) is perfectly well flyable down to stall speed, and below stall speed it doesn't fly at all. For the conditions described, stall speed would probably be something close to 120kn (exact performance parameters are classified). (Some fighters can use thrust vectoring and other aerodynamic tricks to retain a limited amount of post-stall maneuverability but that doesn't really apply in this case.) In an engine failure situation, pilots are trained to trim the aircraft to the speed that will give the best glide ratio and that's going to be well above stall speed.
Vertical flight mode wouldn't be usable in a situation like this. They only transition to vertical flight at low altitude with the landing site in sight.
Every plane has what is called a stall speed, which if you go below it it loses its ability to stay in the air. Above that, even 1kt above, the plane will still fly. Also, to make sure you can fly the longest possible distance to have time to troubleshoot any issues, planes also have what is called best glide speed. If you maintain that, you’re golden.
There are a lot of factors that come into play when you lose the engine, but unless there is a serious issue, you still have control over the flight surfaces.
Vertical flight mode wouldn't be usable in a situation like this. They only transition to vertical flight at low altitude with the landing site in sight.