I think I see what the problem is - GSAP doesn't natively interpolate "vw" or "vh" units (though it likely will when 2.0.0 is released, but that won't be for a few months most likely). It can set() them directly, but interpolating is different. In this case, the inbetween values are converted to px. So here's a solution that gives you the same result but just does the conversion for you at the right time:
Is that what you're after?