SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

// Transform a value into rem
// Assuming baseline is set to 10px on :root/html
@function rem($value, $baseline: 10px) {
    @if type-of($value) == list {
        $result: ();
        @each $e in $value {
            $result: append($result, rem($e));
        }
        @return $result;
    } @else {
        @return if(($value / $value) > 0, if(unit($value) == px, $value / $baseline * 1rem, $value), 0);
    }
}
// Use rem units with px fallback
@mixin rem($properties) {
    @each $property, $value in $properties {
        #{$property}: $value;
        #{$property}: rem($value);
    }
}

.test {
    @include rem((
        padding: 20px,
        width: 300px,
        height: 350px,
        line-height: 20px,
        margin: 0 10px 3vh 0px
    ));
}

SassMeister-output.css

.test {
  padding: 20px;
  padding: 2rem;
  width: 300px;
  width: 30rem;
  height: 350px;
  height: 35rem;
  line-height: 20px;
  line-height: 2rem;
  margin: 0 10px 3vh 0px;
  margin: 0 1rem 3vh 0;
}

Final example returns 0 instead of 0rem
Does not remove the unit for the px fallback