(Off topic but please) RPI VLAN How?

I am sorry for this. But I really am at wit's end

Recently I was going through the sudo apt update and sudo apt upgrade on a working RPI.
It had WAP and a lot of other stuff working.

After the updates, the WAP stopped. It was running Jessie and all/most of the help was "use Stretch". I cut my losses and updated to Stretch.

After a lot of messing around, I got the WAP working. Yeah, I know. Every one says it is easier. But anyway..... I finally got it working.

After a lot of thinking about how I want the system working, I wanted to segregate things.
The "existing" WAP was/is good for some things, but not really nice for other things.

The other things are strictly sensors and stuff. Other Arduino IOT things. They don't need internet access.
Basically they only need to talk to the WAP - MQTT messages.

For the sake of learning I decided to try setting up multiple WiFi networks from the RPI.
I discovered that rather than how I originally thought of it, it was said to me at some stage during the search: this is a VLAN.

I get how to set up a WAP, but making the VLAN device has me stumped.
I get that it will be called something like: wlan0.x

I have sat there for hours trying but failing.
Luckily not as bad as it has been and I had to plug in a monitor and keyboard to get it working.
(But that's another story)

One link adds the VLAN entry in /etc/network/interfaces which is fair enough, but reading what it says at the top of that file about /etc/network/interfaces.d it makes me wonder.

Anyway: Bottom line, I can't get all the pieces together.
What I know is that I have to edit:
/etc/network/interfaces (or /etc/network/interfaces.d/(filename))
There is mention of:


Then it goes on to NAT stuff.

As I don't need NAT, that part is not really needed.

That DAEMON_CONF part has me a bit stumped if I have multiple WAPs.

Anyway, without wanting to bore you to bits, that is what I know.

Quick P.S. about the VLAN is this:
Supposedly to be saved as filename vlans in /etc/network/interfaces.d/

auto eth0.10
iface eth0.10 inet manual
  vlan-raw-device eth0

But I would change that to wlan rather than eth.

It doesn't seem to work.

I've never set that up on a Pi or any other Linux device.

I would recommend asking this on Stack. There are 3 communities that might be of help,

Super User

Q&A for computer enthusiasts and power users

Raspberry Pi

Q&A for users and developers of hardware and software for Raspberry Pi

Server Fault

Q&A for system and network administrators

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I would recommend asking on Raspberry Pi forum itself https://www.raspberrypi.org/forums/

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They - alas - point me to links with varying consistency.

Though I check they are for Stretch, the way of doing it is different on different links.

None of them work.

So it is difficult to know where the problem is.

I get there may be more than one way to do this. But again: none of the ones shown to me work.