This guide will cover the simple solutions first, then the complex one afterwards.
The complex solution is guaranteed to work if you perform it correctly, but is complicated, so I'll explain the simple solutions first. If you are knowledgeable enough for the complex solution, just skip to it.
• Simple stuff:
To fix the failed to find steam error, you should run the game, as well as Steam itself, as an administrator. To do this, Navigate to your Steam directory: (C:\Program Files (x86)\Steam, right click on Steam.exe, and click on "Properties".
Next, click on the "Compatability" tab, then check the box next to "Run this program as administrator", then click OK (as shown below).
Now you'll need to restart Steam in order for the change to administrator mode to take effect. (You shouldn't notice any significant difference). Make sure you save this guide's page while you restart steam so you don't lose your place.
Restarted Steam? Good.
Now you need to do the same thing for the game itself, which shouldn't be hard. Simply navigate to the game's executable file: vampire.exe. It's located in: C:\Program Files (x86)\Steam\steamapps\common\Vampire The Masquerade - Bloodlines
Now right click on the vampire.exe, click on "Properties", click on the "Compatability" tab, and check the box next to "Run this program as administrator", then click OK.
There, that's it for the simple solutions, if these didn't work for you, attempt the complex solution below, but read the directions for it very carefully.
• Complex Solution
Still not working, eh? Don't lose hope, this game is worth it.
Ok, before we start, a little disclaimer is needed, this solution is complicated and will not work unless you get it exactly right, so read carefully, and make sure (like 3 times) that you've done it right.
Ok, so first, click on your Start menu (Windows logo on your taskbar) and then right click on Computer, then click on "Properties" as shown below.
Next, click on "Advanced system settings"
Now click on "Environment Variables..."
You should see at this point a window with the section titled "User variables for [your username]" where [your username] is whatever your actual username for your PC is.
In this section, scroll through the options until you see the Variable called "PATH" (could be lowercase as well). Select the "PATH" variable, and click on the Edit... button as shown below.
Note that if you do not have the path environment variable already, use the button New... instead, type "path" into the variable name text box, and enter your Steam directory as described below.
In the next window, you'll see that a lot of text is already selected. Do not press delete or backspace.
Make sure you now press the "End" key to make sure anything you type doesn't overwrite anything already there. (This is shown in the picture below.)
Make sure you're at the end of the entry by pressing the right arrow key, checking that it doesn't scroll any further. At the end? Good. Now add a semicolon ; Make sure it's a semicolon-> ; and not a colon-> : Dot and comma, not just 2 dots.
Got the semicolon? Good.
Again, if you don't have the path variable, you don't have to worry about putting the directory at the end of the variable value, just paste the following directory into the value text box.
Now just paste the following directory after the semicolon: C:\Program Files (x86)\Steam, and press ok.
Next, restart Steam, and launch the game; it should now be able to find Steam.
Credit for this complex solution goes to valex-jedi
and their original post on the Steam forums
I just paraphrased uncommon knowledge in this field and reposted it in a guide for the Steam Community guide feature, and properly cited the source, as per non-plagiarism standards dictate. Oh, and the screenshots are mine too.