I try to help
you need the wet floor reflection plugin download wetfloorplugin
optional: Octagonal/Quad reshape/matteplugin Download it
1.make a new canvas
2.fill it with the paintbucket :PaintBucketTool: with a grey color
3.use the Zoom and rotate tool in the layer menu to change the perspective to your satisfaction
or use the Octagonal/Quad reshape/matte plugin
4.make a new layer
5.make a curved line with the line/curve tool like in your sample image :LineCurveTool:
6.use the magic wand tool with a tolerance about 12% and click in the room above your line
7.fill this selection with the paintbucket :PaintBucketTool: with white
8.go to the layer properties :Properties: and change the opacity of the layer until your satisfied
9.make a new layer
10. make a retangle selection and fill it with the paintbucket :PaintBucketTool: with your favourite color
11.make a selection under your rectangle and run the wet floor reflection plugin
12.deselect your selection by pressing strg+d
13.and your done !
I make simplly my own vignette:
make a new layer draw a black line around your pic ,the size is depending on how big your vignettte should be, then use gaussian blur with your favourite radius around 40-80