protection against vibration

If by "gull wing" you are referring to this package

then you might get some hints from NASA Workmanship page 8.02 under Conformal Coating and Staking (Bonding) from here

My guess is that under vibration the body of the package is moving out of sync with the pcb vibrations - and with differing amplitudes - therefore stressing the leads and the soldered joints. Bonding the body of the IC to the board in accordance with NASA recommendations may help in ensuring the package moves in sync and with the same amplitude as the pcb vibrations. A thicker pcb should also help suppress uneven modal vibrations in the pcb itself.

package

here

The problem is more likely to be the flex of the PCB than anything else. The resonant frequency of the package vs PCB mas spring system will be very high. You have to get near the resonant frequency before you get large deflections.

A PCB supported by standoffs only at the corners can get into many very bad resonance modes. There is the obvious simple one. The PCB can form an "S" shape parallel to either edge or running from corner to corner in either direction. The standoffs can lean back and forth allowing for modes where the PCB shakes right left, front back and twistwise.

package

here

Sir, thanks for the response, I suppose you are suggesting to put more mounting points in the center. Please elaborate this S shape parallel to either edga. Sorry for being unable to understanding your point...

The standoffs can lean back and forth

package

here

Don't put the extra mounting point exactly in the center.

Trying ASCII art of the edge of the PCB at one extreme of its flex: