Good question: I have had my HTC Vive for a while now, and I have my whole setup complete. (keep in mind that all of these accessories are not required, so don't feel obliged to buy them just to play in VR. Anyway, here are a couple of the things that I would recommend:
Deluxe Vive Audio Strap
This add-on allows you to hear VR audio and also have a nice, snug fit for your headset. It’s definitely worth it.
Alienware M15 Gaming Laptop
Not just any Windows PC will run your VR system well. You’ll need the power of a gaming machine. This Dell Alienware gaming laptop is the right mix of cost (not too expensive) and power (plenty of power to run VR). A powerful computer is crucial for a smooth and high def gaming experience, since the VR headset and software will run from the computer. In my opinion, it’s good to get a notebook instead of a desktop so that your VR setup is portable. However, a notebook is not easy to upgrade, so you might need to upgrade your notebook after a few years.
If you prefer another brand of computer, here are the minimum specs I recommend to give you a bit of future proofing, and also will be able to support your own VR creations, if you decide to start building your own VR scenes:
Graphics: NVIDIA® GeForce® GTX 1080 or better
Processor: Intel® Core™ i7-7820 or better
Memory: 16 GB RAM
Video out: HDMI 1.4 for the Vive, plus a second video out for an external display
USB ports: 2x USB 2.0 port
I recommend the Alienware M15 Gaming Laptop Intel i7-8750H (2.2-4.1 Ghz on 6 cores). This machine meets or exceeds these requirements with a 15.6″ LCD, 16GB RAM, 512 GB PCIe M.2 SSD storage, NVIDIA GEForce RTX 2060 graphics card with 6GB.
Lighthouse Mount Stand Kit
These stands are for mounting the light houses (sensors for HTC) without drilling wholes into your walls.
Leather VR Face Cushion
These leather cushions provide nice padding, important for a pleasant VR experience. The cushions will wear out after a while and also become dirty after several people use the headset for a while.
I found an article with more useful items and links to buy them here: https://grantreviews.blog/2019/04/21/htc-vive-equipment/ and I strongly recommend that you check them out.