MSP430 + VLOCLK + UART?

The VLO is not to be used with anything that requires even just a moderately accurate timing, not to speak about UART communication.

On the 5438, the VLO is somewhere between 6 and 14 kHz, not calibrated, and has a temperature drivft of 0.5%/°C (%, not PPM!)

So the VLO is only to be used if you need to somehow get a clock signal (e.g. for the watchdog, if response time does not matter)  buit don't care at all how fast it is.

If available, the 32kHz REFO is much more suited for UART communication, it ha sonly +- 1.5% tolerance at 25%C and +-3.5% over full temperature range. Which is might be still too much for UART if the other side is a bit picky.

Also, for a good UART connection, you should have a clock that is fast enough for oversampling ( > 16*Baudrate) and then still fast enough to select a divisor with low error, so you don't have to rely on modulation too much. One should think that clockign an UART is a simple thing, but if you want to ensure a stable and (as fasr as possible) error-free connection with various devices, things can be tricky.

There is no problem to use the VLO for SPI communication (well, it's slow then), as it is synchroneous and either the MSP is clocked from outside (so does not need an internal clock, yet the external may not be faster then maximum system clock) or delivers its clock signal to the slave, so it can be any and even unexpectedly changing frequency (within teh range of the slave specs).

For I2C, the VLO is also usable, since it too is synchroneous, but some I2C devices may require a minimum clock speed.