You may want to start out increasing your film development time by 40 to 60% more than what you use for silver gelatin, VDB requires a contrastier negative than say palladium.
You may also find that you do need to increase exposure a bit if you do not have adequate or pleasing shadow detail in your prints. Try 1/3 to 1/2 stop more exposure if you think you need better shadow details in your VDB prints.
Unfortunately people are often advised to make large exposure increases for alt prints which is usually not necessary.
Each alt process has it's own characteristic printing curve so some testing of one kind or another will be called for.
Good luck,
As Don suggests, if you do not have adequate exposure to get good shadow detail in your prints you need to expose more.
However, my experience is that it is more common to over-expose than under-expose when exposing and printing for alternative processes. The fact that you need a much higher contrast negative (requiring longer development time) can increase effective film speed by as much as a full stop. When I plan to develop negatives to a DR of 1.8 or higher I always use a meter reading that is 1.5X the nominal film speed. Although I use an incident meter and take my readings in the shadow I think it might be about equivalent to placing your shadow values on about Zone II. Remember, the additional time in the developer is going to increase shadow density.
If you place your shadows on Zone III, or Lord forbid, Zone IV, this will have negative consequences for shadow separation, especially with print out processes that are self-masking, such as Ware-Malde POP palladium, Ziatype, VDB, albumen and salted paper.
Sandy King