Hi 

Voltmeter has high internal resistance , so if you Connect in series , it shows resistance to the circuit and result in the decreses  of current .

All the current go through ammeter , the whole purpose for making a measurement with an ammeter is to determine current draw through components or circuit .

But if the ammeter takes away the current , reading will be meaningless .

So , it may result in dead short to power supply , blown fuses or toasted componeted occurs.

Thanks