Pour water into Instant Pot® and add trivet. Grease a (6-cup) Bundt pan and set aside.
With an electric mixer, cream together oil and sugar. Mix 2 minutes.
Add eggs and 1 tsp vanilla, one at a time, beating after each addition.
In a medium bowl, whisk together flour, 3 tbsp cocoa powder, and salt.
Slowly add flour and buttermilk to the mixer, alternating between the two until fully combined.
In a small bowl, combine baking soda and vinegar. Give it a quick stir (it will start to bubble) and pour into cake batter. Mix an additional 10 seconds.
Fold in half of the jar of chopped maraschino cherries. Pour cake batter into greased Bundt pan. Create a foil sling and carefully lower Bundt pan into Instant Pot®.
Close lid and set pressure release to Sealing.
Press Manual or Pressure Cook button and adjust time to 32 minutes.
When the timer beeps, quick release pressure and then unlock lid and remove it. Carefully remove Bundt pan using foil sling.
Use a straw to poke twenty holes in cake. Pour cherry juice into holes. Place cake on cooling rack and let cool, about 30 minutes.
With an electric mixer, mix together powdered sugar, butter, remaining 3 tbsp cocoa powder, milk, and remaining 1 tsp vanilla. Mix until light and fluffy, about 2 minutes.
Once cake is cooled, run a butter knife around inner and outer edges of cake. Turn cake over onto a plate and wiggle to remove Bundt pan.
Spread chocolate frosting on top of cake and top with remaining cherries.