How do I handle the password in vsql command in crontab?

If you have the vsql password in the environment variable VSQL_PASSWORD, it works.
Of course, there's a risk in putting:
export VSQL_PASSWORD='<dbadmin_s_password>' into the OS user's .bash_profile or similar.
But there's a collection of tools out there that store an encrypted password somewhere and decrypt it on the fly to put it into a shell variable. Worth exploring ...

If suitable in your environment,, another solution is to have a passwordless authentication method (TRUST, TLS with self-authentication or Kerberos)

In addition to what others said, you can use the -w option on the vsql command line to set the password, but from a security perspective that's probably the worst because command lines are frequently visible / logged in places anyone on the machine can see. Investigating other security options makes sense here, as Serge mentioned.