Vegas Pro is forward compatible unless you are using plug-ins that aren't supported in newer versions. In that case, remove such effects and save a copy for VP 18.

For what info we need to be able to give advice you may have missed this pinned post:
https://www.vegascreativesoftware.info/us/forum/important-information-required-to-help-you--110457/

Media info, a screenshot of project settings and a screenshot of render settings would be especially useful.

By more info, to be specific, we need:

1. Your project settings

2. a mediainfo report for the media used in your project. One report per type of media (so if you have 5 camera angles, but they're all the same camera recording in the same format, we only need one report.)

3. Your system specs including GPU driver version and windows 10 version.

This will get us started with assisting you.

@RogerS @fr0sty do read @yuri-kYIK postings from over 3 years back:

https://www.vegascreativesoftware.info/us/forum/vegas-14-repeated-crashes--105412/#ca652048

IF this is the PC spec then . .

Windows 10 64 bit
Intel Core i7-4790
RAM 16.0 GB
Cpu 3.6 GHz
NVIDIA GeForce GTX745

Yeah, that doesn't meet the minimum system specs required for V17 or 18, if those are still your system specs. That would explain it. Those specs don't even meet the requirements for 1080p, The GPU must be at least a GTX 9xx or later. 4k projects require even better hardware than that.

@fr0sty - Yup. I think we are going to get some unhappy campers here. Thank heavens VP18 NOW has that GPU checker 😉 .

"My System now wont make coffee?!"

"OK.... Check your GPU and report back."

If that's the GPU it is old, however the poster didn't complain about editing or GPU performance.
We don't know the GPU is even involved here- what were the render settings?

If that's the GPU it is old, however the poster didn't complain about editing or GPU performance.

@RogerS - Well, I disagree. Reread his Point 3 and 4. That gave me a clue that thats to do with access to Render and onwards.

 

5. After about four or five hours the process was completed (in Vegas 17 it takes just over an hour)

6. Completed "rendering" contained nothing except - about one hour long blackness.

Rendering the in Vegas 17 took 67 minutes and was what I expected. Luckily I kept back up Vegas 17 project as, based on my previous experience with Vegas, the software is not forward compatible.

 

I don't know the fix, but it would be tremendously frustrating waiting for that period of time and have black frames. Try to look for clues that it's not processing correctly, such as cpu, gpu, and gpu decode. In this example the goose is obviously cooked when I look at GPU Decode graph. I don't even know if your vegas partially crashed or something else. If it's telling you 5 hours again and you know that's not fit for purpose cancel and keep trying other things.

The issue is, if the OP does indeed still have that 7xx series GPU, there are no encode or decode chips on it, so all it can do is timeline acceleration, and not very well.

Grazie where is the GPU checker?
I had to turn my GTX 1080 acceleration Off to complete a render.
Trying to render with GPU On locked V18 many times > Task Manager to close.

@PeterWright Under HELP On the Toolbar.

Thanks - I'm downloading new drivers .....

Still bad. Latest NVIDIA Driver installed. Render with GPU produces black seconds, not just frames.
Switching off GPU and using exactly same template. perfect render, and still didn't take long.

@PeterWright - And now we move onto your System. Is your GPU compliant with VP18? Others have had to bite the Lead!

Grazie I assume the link under Help is Check Driver Updates - I clicked this and it downloaded the latest,
Does updating drivers from Vegas mean the GPU is compatible with Vegas?

@PeterWright - Good question! What are your GPU specs?

1080 is a compatible GPU.

What are the rest of your system specs?

Nvidia GeForce GTX 1080

@PeterWright - You could be out of luck. Need others to chip in here.

@fr0sty Is that also the GTX too?

yes

We need your full system specs, including windows version, CPU, RAM

Also need a mediainfo report on the media you are using in your project.

You could have media that VEGAS is struggling with in your project, so that will isolate if that is the cause (if so, a quick transcode, rewrap, or changing settings to use legacy decoders may fix it).