Create a priority with no SLA

I would like to create an SLA (or two actually) that don't have an SLA attached to them. For example a Priority of 5 - Vendor. These are for incidents that have been moved on or opened up with a vendor. When this is selected I don't want there to be an SLA attached to it.
Is it possible to do this? At first I didn't put anything in the SLA field, thinks this would mean none. Wrong, we were getting dinged right away for that.
I then knocked it up to one year. This calculated fine, but I was told two years would be better. While I've made the changes for this, the calculation is off on these, not really sure the end date time frame being used. Thus, is it possible to have a null value SLA?