If composer shows proc_open() fork failed on some commands:
This could be happening because the VPS runs out of memory and has no Swap space enabled.
free -m total used free shared buffers cached Mem: 2048 357 1690 0 0 237 -/+ buffers/cache: 119 1928 Swap: 0 0 0
To enable the swap you can use for example:
/bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024 /sbin/mkswap /var/swap.1 /sbin/swapon /var/swap.1
You can make a permanent swap file following this tutorial.