You have lost in the court of appeals. But the issue you lost on is a great candidate for Supreme Court review, so you file a petition for certiorari with that issue as the Question Presented. The other side then opposes your petition. Part of their argument is that, regardless of your Question Presented, the case is a bad "vehicle" for the Supreme Court because there is a separate issue in the case--not addressed by the court below--that also bars you from relief. In other words, they are saying something like this: "Who cares about this cert issue--there's a separate problem with this particular case and the Supreme Court therefore shouldn't waste its precious time here, so cert should be denied."
But there is a good chance that this is not a real vehicle problem. The Supreme Court frequently takes cases that involve a separate issue that was not addressed by the court below and that may indeed be dispositive of the underlying relief you seek. When that occurs, the Court reviews the main issue raised and simply remands the case to the court below for resolution of that second issue your opponents keep bringing up.
This concept is the centerpiece of cert-stage briefing I filed at the Supreme Court in 2016 in McNeese v. United States, No. 16-66. The substantive issues are discussed in this post. The Petition raised a circuit split regarding a criminal defendant's ability to seek a sentencing reduction after entering a binding plea. The Department of Justice's Opposition acknowledged the split, but argued that a separate issue would bar my client from the sentencing reduction anyway. Thus, they said, there is a "vehicle" problem. What follows is the text of my Reply, which counters that there is no vehicle problem in that context. The Court ultimately denied certiorari, but the Reply provides a framework to counter the "vehicle" argument.