There are two types of vectors you can use. Pure solid color objects and grainy mono-color objects.

You want to create a good color scheme. Use vivid retro colors, mix around with opacity.

You want to almost create a cut and past look to your art.

Here are some reference images I used: giraffes, city, trees, shoes

 

Trees, giraffes, shoes.

You want to cut the objects out of there picture using the polygon lasso. It can be far from perfect.

 

 

Then bring these over to the current document. Go to Image>Adjustments>Threshold.

 

Now you want to bring some elements out but lose but not make the image grainy. Look at my examples. To colorize, go to Image>Adjustment>Hue/Saturation. Click the colorize box. Bring up the lightness and saturation slider. Change the hue to your liking. You can get away with flipping the axis, resizing and changing the shade for duplicates.

 

 

Bird

Basically use the brush tool to color the entire bird one color and import it to the document.

 

 

City

Cut out the city from the sky. Import it to the document. Go to Image>Adjustment>Hue/Saturation and click the colorize box to colorize. Go to Image>Adjustment>Brightness/Contrast. Slide the Contrast slider to the right.

 

 

 

 

Retro Lines

 

The solid lines are just pen stroke and Stroke Pen Path with the appropriate brush.

 

 

 

 

Sky

Use the Brush Panel to change these settings. Take a any brush, make a large spacing. Change opacity setting, scattering. And brush the sky.

 

 

You can also create the appearance of a skyline with a square brush. Just make the brush a shade of black. I felt it wasn't necessary because I wanted it to take away from the trees and giraffes.

 

Final Result

Discuss the Adobe Photoshop tutorials on the forums. You can find the photoshop files to download there too.