There never was a mid 2012 MacBook Pro with 1.5 GB of dedicated video RAM.
The 2.3 GHz i7 processor had 512MB of GPU memory. The models that were higher specced than that had 1GB of dedicated video RAM.
Here's what you're wondering about specifically: I have the 2.3GHz model as my daily computer, typing from it to write this message. When I am not connected to an external monitor or using intensive applications it uses 1536MB (~1.5GB) of RAM from the normal system memory and completely shuts down the GPU, saving energy. The higher end model does just the same, using the 1GB of GPU memory only when needed.
They all use 1.5GB of system memory if the GPU isn't on. When it is needed they seamlessly turn on the more powerful, faster graphics dedicated memory.
Here are three screenshots I just took... First is an overview of the mid 2012 MBP product line, the second and third are screenshots of when my MBP is connected and also disconected from my external monitor. When display is connected, GPU is on.
For what very much most people use computers for, these mid2012 MBPs are still very nice machines.
Since this is my 1000th post, I thought I'd make it a quality one! Happy Monday, hope this helps!
I'm sorry, I was thinking of the 15 inch models. They have quad core processors and GPUs. The 13 inch models have dual core processors and also do not have dedicated GPUs. The app that I am used for the first screenshot, Mactracker, available in the Mac App Store for free if you already have a Mac, does not explain how either of the levels of 13 in MBPs could have 1536MB of shared video ram. It just shows up to 1 GB of shared RAM. I'm not saying the App is infallible, but I have no reason to doubt wither the app or the screenshot you provided.
What does seem curious to me is the the computer with less overall system RAM gave up more video RAM for sharing. 1536 MB of shared RAM for the Intel HD Graphics 4000 onboard video lines up with mine, so that makes some sense. Really, I would expect the shared amount of video RAM to be based off of the processor, but since both are the higher end 2.9 GHz dual core i7 instead of the lower end 2.5 GHz dual core i5, they should be equals.
My 15 inch MBP starts up the dedicated graphics card when I open programs like Google Earth, Aperture, or video games, and also with an external display. It works great for me... When it is using the integrated graphics, it also works well, but then again, in those situations it doesn't need gobs of power in the graphics department. I frankly don't know how well it would work in more intense situations without a dedicated card. The 13 and the 15 inch computers all can drive the same size displays, so yeah....
Here's the screenshot of information for the mid 2012 13 inch MBP...
Your second system is an older version of OS X - can't tell from that shot, but probably Mavericks --- might even be Lion, which is original version to that model.
El Capitan will provide a higher amount of video memory from the same Intel HD graphics.
Lion would show the 512MB that you see. Mavericks (I think) would show 1024 MB, and Yosemite, then El Capitan shows the 1536MB. (But, I thought that you would also need more than 8GB to display the full 1536MB. I guess I never tried taking any RAM out, but there it is in the screen shot showing 4GB of RAM. )
Anyway, the two systems don't have the same OS X version. That's the difference.
I remembered that my mother also has a mid 2012 MBP, hers is the more relevant to this thread 13 inch model. It apparently reports that it is a 2.5 GHz 13" MBP has 10GB of system RAM and shares 1536MB of it to the integrated Intel HD 4000 processor. SO, if you have any mid 2012 MBP with the most up to date operating system, it will at least have 1536MB of (shared) video RAM, or better if a 15" model. I think this just about solves it.