I got a sunburn several months ago on vacation, and the outline is still there. Should I be concerned?

No. Discoloration is common after a sunburn. Different skin color will react differently---the darker your skin, the more likely this will happen. If you are out of the sun(winter), the difference in skin color will last until you get sun to even out the pigment. Do not worry.
No and Yes. About the outline? No -- it will go away eventually. About the sunburn? Yes - do it repeatedly and you will accelerate the aging and wrinkling of your skin, and increase your risk of developing skin cancer. Buy a good broad-spectrum sunscreen and apply it every day, whether you plan on sunbathing or not. Your skin will thank you in 20 years.