Is using a secondary dominant in a chord progression similar to using modal interchange? I hear the use of bVI chord in a major key quite similar to a V/V chord (It might just be me). How are they different?
A secondary dominant, by definition, leads to the dominant. Thus V/V, in key C, it's D(7) leading to G. Going anywhere else, that D or D7 isn't going to be a secondary dominant.
The bVI you quote is, in key C, Ab7, which can be considered as the V of a new key, C♯/D♭. A common key change in many songs. If it came with G straight after it, it could be considered as a sort of strange cadence, either imperfect if it stopped after playing G, or perfect, (G>C) with the A♭ as a red herring.
They both contain a 3rd and b7 of each other, but swapped over, producing a kind of tritone substitution effect, which may be what you're catching. That said, their functions are different.
You've discovered the 'b5 substitution' or 'tritone substitution'. Yes, in the key of C, Ab7 can function as V of V. It shares a tritone interval with D7 - the notes C and F# (Gb).
This illustrates the general principle that any dominant function chord can be substituted with a chord rooted a b5 higher. If both notes of the tritone are present (i.e. if it's Ab7 rather than just Ab) the dominant effect is heightened, but just as G (without a 7th) is still the dominant of C, Ab can still sound like the dominant of G.
On a more pragmatic level, a 'dominant' can be rooted a 5th below (G7 to C) or slide down from a semitone above (Db to C).
Common Practice harmony used this idea in the Augmented 6th chord, normally used as a pre-dominant. Jazz-influenced harmony uses the b5 substitution freely.
I'm not a great fan of 'Modal Interchange'. It does little beyond hanging a 'permitted' label on some 'safe' chromatic chords. We have a good, firm functional reason for tritone substitutions!