I have not beaten the game!
I'm still playing through the game, so for all I know the requirements could get stricter on later levels. Furthermore I am going off of observation, meaning that I could be completely wrong about the workings of the Rank System.

Please do slide a comment if I need to change anything! I made this because I was a bit confused about how V Ranking worked, and I want other people to have an easy way to clear up their confusion.

For a stage to get the V rank, you need to meet the following requirements on it in a given attempt:

• The difficulty MUST be set to hard, else meeting these requirements will only give an S+.
  
• 100% of tiles painted
  
• Obtain both Bonus Fruits
  
• Bonus Phase Start/Stage Clear Power need to be around or higher than 50%
  
• A longest tile streak totaling to around over 17% of the stage
  
• No lives lost

In other words, beside "Total Score" under Rank Explanation, that number needs to be 100 on Hard Mode. Note that you do not need a maximum tile streak of 100% to get a V rank on a given stage.

Actually obtaining the rank is easier said than done, so here are a few more specific requirements...

Tiles Painted
The entire stage needs to be painted to the target color. This does mean on stages where titles have to be stepped on multiple times that even if every tile is stepped on once, V or S rank will not be given.

Fruits Picked
Both fruits need to have been spawned and obtained. Do note that all spawned objects, including bonus fruits, disappear when going into Bonus Phase.

Bonus Start and Stage Clear Power
The power meter should be as full as possible when entering Bonus Phase. While taking damage isn't the end of the world here (at least as Cerise), it's better to try not to take damage when nearing a majority of the stage painted. Note that both do not have to be at 100%, and it appears that Start Power is weighed more than End Power.

Longest Tile Chain
This is one of the more confusing ones. Unlike other score categories, you do not need a high percentage; I've been able to V rank when the Longest Tile Chain was only 17%. Still, be mindful of how you plan your route across the stage. 20% or higher should be your target.

Losses
This comes without saying but not taking damage is probably the most important part of this. As previously mentioned, Cerise can tank a hit near Power 3 in the latest update without pulling up the "Oh no!" screen, which does not count as a Loss. Do note that this will also drain your power meter quite a bit, so you may need to attempt the level again if this happens near the target percentage.

While you could be losing points in any of these sections, I've usually found that the hardest to meet the requirements for are the Fruits Obtained and the Longest Tile Chain.

Not picking up one of both bonus fruits is enough to drop the rank down to S and A+ respectively, and tile chain is fairly easy to miss points on accidentally due to Enemy Paths or your keyboard/controller straight up not working properly!

Don't give up though! Look up tutorials on how to play better, practice, and you WILL be able to get that V rank!