I see Vause as the brick wall Piper has to slam her head against.
She's an in your face reminder about her crime and why she committed it. Not because she was hungry or homeless or drug addicted and needed her next fix.
Piper broke the law because her lover asked her to do so.
Without Alex around, I think it would be easier for Piper to rationalize it away as "youthful indiscretion" etc, something not "her" fault.
But Alex is a living breathing reminder that while she personally only committed one crime, Piper enjoyed the fruits of the drug trade
for (?) 2 years before finally bailing on the relationship.
Alex is also a wonderful bookend to Larry (dangerous vs nebbish) and quite frankly unless one or both grows up/matures, neither is a good match for the woman who's drawn us into this world. (Presuming, of course, that PIPER grows up.)
Here's a great article with LP that was conducted just before the current internet wailing/gnashing of teeth over her future with OitNB.
See, TV guide
agrees with me. Kind of.