Hi Yu Huixuan
As you can see in module operation in low power modes, UART module is just enable in VLPR and VLPW modes, so in VLPS mode, uart module register states and associated memories are retained but it is not working:

So in VLPS mode it doesn't matter that you configure your baudrate it will not work until you set your device in VLPR or RUN mode.
Hope this information helps you
Have a great day,
Jorge Alcala