According to a page on the site of (in Dutch) the Royal Dutch Touring Club, you're required to have a warning triangle and a safety vest. While a fire extinguisher and first aid kit are only required for Belgian drivers, they still recommend taking them as well. Another page of theirs (in Dutch) which gives an overview of these rules for various European countries says that local police does not always take into account that some rules don't apply to cars registered in foreign countries.
Legally speaking there is a difference between rules that apply to the car and rules that apply to the behavior of the driver. The requirement of having a fire extinguisher and first aid kit are legally specified as rules that apply to the car, and therefore only apply to cars registered in Belgium. As for the warning triangle and safety vest, according to a discussion in the “nl.auto” newsgroup (in Dutch), the Belgian law does not state that these are required to be present in a car, like the other two items, but instead specifies that when leaving the car in the middle of the road, you need to be wearing a safety vest. If you have an accident, you're required to put out a safety triangle. So you're not technically required to have these items in the car, but it would be difficult to follow the law otherwise. For the safety vest this was quite literally explained in a text accompanying the relevant Royal Order:
La modification du Code de la Route ne s'accompagne pas d'une adaptation de la réglementation technique des véhicules. En effet, si la veste réfléchissante était imposée comme équipement obligatoire du véhicule, seuls ceux immatriculés en Belgique seraient concernés. Il est donc plus logique d'introduire cette obligation sous la forme d'une règle de comportement même si l'utilisation de cette veste suppose nécessairement que le conducteur en dispose d'une dans son véhicule.
Translated, this says:
The modification of the highway code is not accompanied by an adaptation of the technical regulations for vehicles. Should the safety vest be imposed as an obligatory equipment of the vehicle, then this regulation would only apply to vehicles registered in Belgium. It therefore makes more sense to introduce this obligation as a rule of behavior, although the use of the safety vest necessarily implies that the driver has one in his vehicle.
Edit: It has been pointed out to me that Belgian law does also have a rule for safety vests which applies to the car. This is the result of a Royal Order that did introduce an adaptation of the technical regulations for vehicles. It was issued about 2 years after the above one, in 2009. As they stand, there is now a behavioral rule which applies to anyone driving in Belgium, and a car equipment rule which applies to any car registered in Belgium. There is no accompanying text for the 2009 Royal Order which explains why the extra rule was introduced after all. I suspect the first one did not give enough legal basis to fine people for not having the safety vest in the car, as it only “implied” you needed to have one. As far as I know, the requirement of having a fire extinguisher and first aid kit are (still) only specified as rules that apply to the car.
However, in summary, rather than trying to argue the finer points of these legal distinctions with local police, I would recommend following the advice of the Royal Dutch Touring Club by simply taking all four items. It avoids the need for discussion, the possibility of fines, and they might be useful if you do have an accident.
