If an employee follows all the VTO rules/policies, then no, you really should not use it as cause to fire them.
If you do not have a VTO policy, and the employee takes PTO to do volunteer work and does not violate the PTO policy, you should likewise not consider that a terminable offense, as there is no offense.
However, if a VTO policy is breached—for example, if an employee misrepresents the nature of the volunteer work by working for compensation at the organization, lying about what organization they will be serving, etc.—then, like any other policy violation, your company may use your normal disciplinary process.