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Dear Sandy;
If PBr 7 (natural iron oxide) is the only pigment in there, it should be just fine. That is the same pigment used for the natural umbers (when manganese is present) and of the siennas (when there is no manganese). The calcined version (calcinating is the process of cooking the pigment to oxidize it) the pigment is used for the Burnt colours, and the un-calcined are the Raw. In all events, these are considered very stable. Heck, it's just dirt, and dirt lasts a very long time. On occasion, I hear claims that the umbers "slip", as you say, or are somehow fleeting, but I don't know where those sentiments stem from. Any literature on the iron oxides, either natural (umbers & siennas) or synthetic (Mars colours) is pretty firm in stating that they are solid, lightfast, and permanent.
The downsides to umber pigments are that they can promote the "sinking-in" effect on colours mixed with them and they can be weak tinters, especially raw umber. It is this weak pigmenting strength that probably leads to the mistaken conclusion that they are transient. By contrast, the Mars colours seem to generally be strong tinters. They do not contain manganese, so they dry more slowly, too.
Anyway, the bottom line is that your tube of vanDyck Brown should not present any problem if all it contains is PBr 7. It was a good buy, although it is probably falsely named.
All the best.
Juan
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