If you are having weird random problems it may be due to you system running out of resources due to corrupt saves, other script intensive mods (like Sim Settlements), or leftover proceses from removed mods. To see these you need to enable debug logging as outlined below.
You will then get a debug log file everytime you play the game: "My Documents\My Games\Fallout4\Logs\Script\papyrus.0.log"

Note: If this appears to be an issue with this mod the Author will instruct you to install the 'debug' version of the mod as well. In this case you will also get a second debug file: "My Documents\My Games\Fallout4\Logs\Script\User\FPVertibird.0.log" (note the "user" subdirectory)

Enable debug logging:
- in your "Fallout4Custom.ini" file in your "My documents\My Games\Fallout4" directory add the following lines to the end

[Papyrus]
bEnableLogging=1
bEnableTrace=1

NOTE: If you use Vortex/MO2 then you need to make these changes via their special options (as editing ini files directly does not work in their case).

Optional - Install debug version of this mod ONLY DO THIS IF THE AUTHOR INSTRUCTS YOU TO!
- If this a 'hotfixed' version (such as 1.6.3-6)  then install the DEBUG file (under Old Files), otherwise ...
- Via your mod manager, 'reinstall' this mod but this time select the 'Debug' version during install.
- If your mod manager does not support this then uninstall the mod and reinstall it (DONT restart your game while the mod is uninstalled!)

Recreate the problem:
- restart your game and do whatever is the issue with the vertibird.
- end your game
- now look for debug log file(s) for errors from this mod (lines containing "FPVertibird")
- this has the debugging information for me

To know if the FPVertibird mod is actually running and correctly installed:
- Check your Papyrus.log.0 for line(s) starting INFO - FPVertibird, one saying "using F4SE V0.6.x" will only be there if F4SE is correctly installed.
- For each FPvertibird you MUST get a line "INFO - FPVertibird [fpvertibird:fpvertibirdscript < (FFxxxxxx)>] found. Version: 1.633000 State: x.xxxx"
- The reported version (or upgraded version) should be the same as the version of this mod you installed. V1.6.3-3 reports as 1.633000.
- Report any Warning or Error messages the the author.

Remove the debug:
- (if you installed the debug version) Uninstall the mod and re-install it without choosing the debug version (DONT restart your game without the mod installed).
- undo the Fallout4Custom.ini file changes (otherwise you leave debug logging on).


Looking for problematic Mods

Download and install the excellent "ReSaver" program (fallrim tools) and run this on your most recent full save (not an Exit save).
This will tell you about issues in your save (normally due to uninstalling mods mid playthrough).
I would NOT recommend 'cleaning' your save unless you are prepared to experiment. It may corrupt that save.

However, I do suggest you look at the "Active scripts" section - there shouldn't be any.
If however there are, then these will be hogging your resources all the time - especially if they are 'unattached' (from an uninstalled mod).

There is a lot you can do here but I am not able to give you further advice - there are lots of that from more informed people out there to find...