Tacx Vortex: I've lost speed output following a firmware update.

The old version of the firmware can be installed using the old versions (but not the latest version) of the Tacx utility app:
https://apkpure.com/tacx-utility/tacx.android.utility/versions
Don't forget to turn off automatic app updates.

Two ways to use the Tacx utility app.

First way. Through firmware file with .tacxfirmware extension. For example, for Flux (S), Flow, Vortex, Genius and Bushido - through this file:
http://download.garmin.com/support/Fitness/Tacx/Flux_S/restore.tacxfirmware
Use this instruction by analogy (keep in mind that Neo 2 and firmware for Neo 2 are mentioned there):
https://tacxfaqx.com/knowledge-base/neo-2-firmware-0-0-18-rollback/

The second way. Install such an old version of the Tacx utility app, inside which there is the required firmware version.

Thanks Alex, I've clicked the link but it asks me to Use APKPure App to download the utility app. Is this App safe to use?

The security of links and files is easy to check with this online antivirus aggregator:
https://www.virustotal.com/gui/home/upload

There is no need to download and install a special APKPure App, since the installers for the Tacx utility app are presented in the form of standard APK files.

Thanks Alex, I can download the Tacx firmware zip file - onto my phone but that's as far as I can get. When I tap the file the Tacx utility zip I don't see an option to open the Utility App and if I extract any of the files in the zip I get "search the playstore" message for a relevant app. There must be a way of opening the utility app and rollback the firmware file - I just can't seem to do it. 

It is possible that the file extension has changed during the download. There should be no .zip extension. The file name should be restore.tacxfirmware.

In addition, you can use the second method, which is easier about the execution procedure.

Thanks for your help Alex, I've tried all the combinations of firmware rollbacks etc which you kindly suggested. None of them successful. I can only assume that Garmin have decided to 'kill off' Vortex turbo trainers via the firmware update. That's me finished with Garmin. I'm off to buy a Wahoo instead!

FOCUSEDSOFT said:

None of them successful

It's not entirely clear from your post what exactly was unsuccessful. Did you fail to rollback the firmware or did the firmware rollback not fix the problem?

The restore.taxcfirmware file option simply installs the latest version of the firmware - so that's no good.
I've tried also installing various older versions of the Tacx Utility App - the firmware upload to trainer process starts OK and then halfway through I get a "waiting" message, followed by a message telling me to unplug the trainer mains cable for 5 seconds - then the process repeats again.

The restore.taxcfirmware file contains version 3.1.22/0.4.3. Is this the latest version for Vortex?

You can also try the old Windows utility (from TTS4) that can update the firmware for one of the two Vortex Smart processors:
C:\Program Files (x86)\Tacx\TacxTrainersoftware4\updater\TacxUpdate.exe

If you do not have TTS4 installed, then this utility can be downloaded separately here:
https://drive.google.com/file/d/1b5mdUzx05YcaqpMHQUSwmH7ALOmfZ1wA/view

The firmware versions for Vortex can be found here:
https://drive.google.com/file/d/15FuCzenzBD93uRCLI6yxNGhqr_5hJ6Md/view

Before using the utility, turn off the internet and place the trainer as close to the ANT+ antenna as possible.

I would also recommend that before this operation, disconnect the right side (electro unit), completely, including a contact wire, and then attach it back. Sometimes this action helps to revive the device.

Thanks for the info Alex, I believe the 3.1.22/0.4.3 firmware is the latest version. Or at least that's what my app is telling me.
If I download TacxUpdate.exe from google drive and install on a Windows pc do I also download the formware files https://drive.google.com/file/d/15FuCzenzBD93uRCLI6yxNGhqr_5hJ6Md/view as well?
And if so, where do I copy the files to once downloaded?
Regarding "Before using the utility, turn off the internet and place the trainer as close to the ANT+ antenna as possible." 
Do you mean place the Windows Laptop, with bluetooth switch on, close to the trainer?