I want to share here at least three different options on how to compensate this limitation.
Option 1: Using the vrimage and EXR output. I have done a tutorial for this one HERE.
Even with this with higher settings I still crash my Vray Su.
Option 2: Using render region. Here is how I do it.
Step 1: Render in low resolution and small output size. Here I used low Image sampler and output first
Step 2: Crank up the Irradiance Map and Light Cache settings. See the render...
|Save LC map|
|Save Irradiance Map|
|Increase Image sampler|
|Load Maps again|
Step 4: Increase the Vray output.. Here I will do 3000 x 1403px. Hit Render. Now wait for the Vray Frame Buffer to appear once it appeared stop the rendering.
Step 5: Use the render region
In this render I selected the 1/4th part of the render image in the frame buffer.
Option 3: Use Image Enlarger.
There is a free program that you can download HERE called Smilla Enlarger. For me this is a very good alternative. It's faster and at the same time simpler.
Produce a smaller render output, then enlarge it using this software.