The general rules for downtime activities allow spending coin to improve the roll:
For any downtime activity, take +1d to the roll if a friend or contact helps you. After the roll, you may spend coin after the roll to improve the result level. Increase the result level by one for each coin spent. So, a 1-3 result becomes a 4 or a 5, a 4/5 result becomes a 6, and a 6 becomes a critical.
For the Acquire an Asset, Reduce Heat, Recover, or Long-Term Project activities, this makes sense: they have various outcomes keyed to each of those results (1-3, 4-5, 6, critical).
But how does this work for the Indulge Vice action, where the outcome (stress removed) is simply equal to the highest die roll? Suppose I roll a 1, and want to spend coin to improve it. I could see this working three different ways:
The next result level above 1-3 is 4-5, so I can spend a coin to increase it to a 4 or 5. Lucky me.
Indulging a vice doesn't use the same result levels as everything else; it has 6 discrete levels, so spending a coin increases a 1 to a 2.
Indulging a vice doesn't use this mechanic at all. I get my 1 stress relief. If I want more than that, I have to take the Indulge Vice action again.