Can I use an illustration of a classic volkswagen beetle or bus as part of a design on a t-shirt that I sell?

Legally, VW can definitely stop you from using their designs and trademarks, but there are shifting tides within trademark law that question aggressive enforcement of trademarks. What this means to you is that VW may possibly be part of the shifting tide, and perhaps make them less likely to immediately pounce on all uses which could merely be perceived as infringement. To be clear, blatantly selling t-shirts with VW across the front is not acceptable and would face certain legal consequences. But some professionals would agree that there is a sliding scale between blatant infringement on one end of the spectrum, and acceptable incorporation of a brand into a piece of art or a design. The goal of brand managers is to protect their brand, and the incentive to take legal action can be lessened were the use of the brand is not only not deleterious to a brand but perhaps even celebratory, a smart legal team may opt to do nothing or have you adjust your design rather than bringing a full frontal legal assault. I think the heart of my answer here is: can you get sued for using some VW branding in a tshirt? Yes. Are you likely to get sued? It depends how you're using the VW branding, and a lawyer can guide you with that. Good luck!

If the Vw you intend to use is the same design as that found on the automobile, the you intend to use a famous mark. The fact that you are using it in a different channel of commerce does not put you in a safe zone. Volkswagen protects its trademarks rigorously and they will likely pursue the matter.

An illustration of a car is Copyright which requires that you obtain permission of the Copyright owner to use the illustration. The letters VW are Trademark. Your use is probably not to help you sell VW's. But VW might sell teeshirts in addition to cars. If so then your use would be Trademark infringement. See Intellectual Property counsel.

You are getting shut down because you are engaging in trademark infringement and copyright infringement. You cannot refer to the brand name Volkswagon, or VW on your T-shirts without a license. Further, the images of the classic beetle and bus are, themselves, protected as trademarks. Without a license, you have no right to refer to this brand in any way, whether words, images or otherwise. Also, the images that you are using probably violate copyright law unless you a permission to use them from the copyright owner. With respect, if you are in the T-shirt business, you need to gain a much better understanding of IP law. These questions are "no brainers"----you can't do this stuff and anyone with even a basic understanding of IP law should understand this easily. I urge you to either educate yourself on IP law basics or get out of the T-shirt business. Otherwise, you are soon going to face a financially disastrous law suit. Further, use of the Volkswagen brand in this manner creates countefeit goods that could even subject you to criminal prosecution. The best way to educate yourself on IP law is to retain and develop a close relationship with an IP lawyer. Your investment in IP counsel will save you enormous amounts of money and aggravation in the long run.