Dont really understand why it works at all. On collision the squared speed should be close to zero.
I dont know how many VulneraBox's you have.
Assuming there are more then one ....
I would:
Give the VulneraBox a instance boolean 'CanCrush'.
VulneraBox > Compare velocity ... overall velocity .. > .. 400
_______ VulneraBox > Set boolean 'CanCrush' .. true
VulneraBox > Compare velocity ... overall velocity .. <= .. 400
_______ VulneraBox > Set boolean 'CanCrush' .. false
VulneraBox on collision with TM_box
VulneraBox > Is boolean instance variable set ... 'CanCrush'
________TM_box > destroy