Ok, I'll go all mathematical for a second here:
A vector is defined by direction * distance.
Legend:
A = Distance
B = Direction.
Distance is a magnitude, which is a constant.
Direction itself is a vector which has X and Y values hereby known as BX and BY.
Bx = X in direction
By = Y in direction
C = controller input in X
D = controller input in Y.
DI can change direction, meaning that in the following formula (A * B = DI), it can only change B.
This can happen by adding X and Y values to the direction. (It can also be done by multiplying by a rotation matrix, but let's go with the simple version of just adding values to X and Y).
Let's assume that the % of the value added is J.
J = Constant from 0-1 which represents the % of DI.
Let's calculate both the X and the Y value for DI, hereby known as DIx and DIy.
DIx = X in Vector after DI.
DIy = Y in Vector after DI
As a whole we get the formula for DI as:
(A * (Bx + (C * J))), (A * (By + (D * J))) = (DIx, DIy)
This is equal to: ((A(Bx) + ACJ), (A(By) + ADJ)) = (DIx, DIy).
As we know VI doesn't actually change the formula, instead you are just adding your own vector! This vector you are adding is actually a % of the original vector.
Let's assume that this % is represented as K.
K = Constant value from 0-1 representing the % of VI.
Let's calculate both the X and the Y value for VI, hereby known as VIx and VIy.
VIx = X in Vector after VI.
VIy = Y in Vector after VI
The formula is then equal to: ((A * BX + (K * C * A * Bx)), (A * BY + (K * D * A * By))) = (VIx, VIy)
This is equal to: ((A(Bx) + KCA(Bx)), (A(By) + KDA(By))) = (VIx, VIy)
Now that we have both formulas, let's go ahead and analyze what you guys are claiming is the following:
You are claiming that VI is always greater than DI or in other words that VI>DI.
In formulas sake this is VIx>DIx and VIy>DIy
Substituting we get the following:
A(Bx) + KCA(Bx) > A(Bx) + ACJ AND A(By) + KDA(By) > A(By) + ADJ
Let's eliminate the variables that are repeated on both sides (A(Bx) and A(By)) by subtracting them from both sides.
We now have:
KCA(Bx) > ACJ AND KDA(By) > ADJ
Now let's eliminate the variables that are multiplied on both sides by dividing both sides by said variables (AC and AD)
Our formula is left as:
K(Bx) > J AND K(By) > J
Since we already know both J and B (both values of B), let's make the formula around the value we don't know which is K.
K > J/(Bx) AND K > J/(By)
And this is the final formula.
What does it claim?
It claims that in order for VI to be stronger than DI then the value K which is the % of knockback in VI must be BOTH greater than J/(Bx) and J/(By). J/(Bx) is the max percentage of DI on X and J/(By) is the max percentage of DI on Y. Basically if a move has 1000 units of knockback and you can DI 300 of it on X and 200 of it on Y, then VI must be able to shift you 300 otherwise DI will be stronger.
Since no one knows just how much VI shifts you just yet this is completely unprovable.