No problem
I'll try to clarify. It works like this:
All UHD Bluray titles have a mandatory HDR10 layer, that's part of the specs. So if your equipment can play HDR10 (i.e. all devices in the chain are HDMI 2.0a compliant), all UHD bluray titles will play in HDR using the HDR10 layer and its associated static metadata.
If a title supports Dolby Vision, it has the HDR10 mandatory layer and its associated static metadata, and an optional Dolby Vision layer (1080p resolution only) which, when played in Dolby Vision, brings the bit depth to 12bits instead of 10bits for HDR10. It also uses dynamic HDR metadata which doesn't use HDMI 2.0a as it's not the same HDR metadata as HDR10 (which is static).
If you want to play this Dolby Vision title in HDR10, you need all the devices in the chain to support HDMI 2.0a. There is no need to support Dolby Vision at all in any device.
If you want to play this title in Dolby Vision, you need Dolby Vision specific hardware support in both the source and the display, but because it uses its own metadata and it uses its specific hardware in the source and the display to negotiate a handshake and switch the display in HDR when HDR content is detected, it doesn't need HDMI 2.0a anywhere. However, all the devices have to support BT2020 content, otherwise it won't work, HDR or not. This likely mean HDMI 2.0 at least, but there might be some exceptions (for example if a Radiance Pro in the chain converts the HDR BT2020 content to SDR P3 or Rec709 depending on the capabilities of the display, when this feature is added, and we don't know yet if it will support Dolby Vision or not).
In practice, most recent and future devices supporting Dolby Vision will also support HDMI 2.0a, therefore HDR10, but HDMI 2.0a is not a requirement to passthrough Dolby Vision content because it doesn't use the same HDR metadata as HDR10 (dynamic vs static). It really doesn't care about HDMI 2.0a as it doesn't use it at all. For example, I think some of the early Vizio Dolby Vision displays support Dolby Vision but not HDR10 because they don't have HDMI 2.0a support (again, Dolby didn't want to wait for these to launch Dolby Vision). These Dolby Vision displays won't be able to play HDR10 titles in HDR, but they will play Dolby Vision titles in HDR. Does your head hurt by now?
We'll have to wait for HDMI 2.1 for HDR10 to support dynamic metadata, and we don't know yet if this will mean new silicon (hence new hardware) or a simple f/w upgrade (which might be possible on high end consumer equipment like Radiance Pro or D&M X7200WA or 8802, but not on Sony or JVC projectors for example).
One important thing to note as well is that unlike with streaming which can offer an compatibility layer with SDR, there is no SDR layer on UHD Bluray titles with HDR (not enough space on the discs, especially on discs with Dolby Vision which need room for the optional DV 1080p layer), so if you play the title in SDR BT2020 it's an on-the-fly conversion. If you can't get the wider gamut (at least P3 or close to P3) and the higher bit depth, it's probably a better idea to upscale the bluray, unless you need the immersive audio track.
Hope this helps