Vivid Unit comes with an M.2 connector and you can connect an NVME SSD to it.
SSD will give you much bigger storage space (Vivid Unit has 32GB eMMC storage) and better read/write speed. However the advantage of booting from SSD instead of eMMC is rather limited -- you will only see one or two seconds faster on boot. It is usually acceptable to let the device boot from eMMC while using NVME SSD as extra storage.
If you do want the device to boot from NVME SSD, you can migrate the root file system to SSD. Below are the instructions.
sudo gdisk /dev/nvme0n1p1
Type 'o' (create partition) and then type 'n' (new partition), press ENTER to use the default sector parameters (this will create one partition for the whole NVME SSD). Type 'w' to save the partition data.
Run this command in your Vivid Unit to download the script:
wget https://www.vividunit.com/download/scripts/change-rootfs-storage.sh
Then you can