Your van is too old to enter a LEZ. The first penalty is a ban on delivering to this low-emission zone. However, it is still possible to enter, as there are "passes" to access an LEZ for one day. Note: There is often an annual limit (maximum 8 times). This requires advance registration and payment. The amount in Belgium is €35 per day.
When you enter an LEZ with a non-compliant utility vehicle (i.e. that does not meet the minimum standards set by the authorities), the surveillance cameras will record your registration number and issue an administrative fine. The amount varies depending on the municipality and the region. In Antwerp and Ghent the fine is €150 for the first violation, then €250 for the second violation and €350 for each subsequent violation. In Brussels, the fee is set at €350, but it can only be collected once a quarter.
There are, however, derogations for special vehicles, in particular those dedicated to the transport of disabled persons. You usually need to register to take advantage of this special authorisation.
Official sites for detailed information, vehicle registration, payments and exemption requests: