you need to tell us what OS and ver. of lmms you have this problems with.
Loading vst's can make lmms crash.
I posted a reasonable method on the mailing list yesterday:
(works for win32, lmms v 0.4.5 & 0.4.6)
*SAVE PROJECT! (better save than sorry..)
*load a default EMPTY! vst from the 'instrument pluging' reposetory
*bring this new empty vst up -eg, activate it /give it focus
*browse the 'my presets' and find the saved vst-instrument you like to load
*RIGHT! click it
*choose 'send to active instrument bank'
-the saved vst-instrument will load in the empty vst-default, in very short time and withour crashes-
Hope that will help you
BR.