Same difference as 'washroom' and 'toilet', i think.

The difference between 卫生间 and 厕所 is similar to that between “pass away" and ”die"

it is the different culture between place,just like subway and underground.

卫生间lavatory,restroom,washroom,bathroom, more indirect,厕所

we usually use them without different! but 卫生间 can be used to wash your hand, make up and so on while 厕所 usually is only used to solve your physical problems!