^ Well, the particular point about using Las Vegas before entering the city limits is not really an issue. That tries to draw the distinction between what most people think of as Las Vegas and the physical city limits of Las Vegas, a distinction that all tourists and most local residents don't even realize exists. (I'd estimate that 75% or more of Las Vegas visitors never actually enter the city limits, but you wouldn't know that without being somewhat informed.)
As to the I-215 sign: There is a supplemental sign just past the westbound onramp from the airport connector that says "Las Vegas // Strip/Downtown // Use [I-15] North". With that sign there, SLC can easily be used on the exit direction sign instead of Las Vegas. Note that eastbound 215 does use SLC as the control on the advance and exit signs.
I think I-15 north should be signed for Salt Lake City anywhere within the Las Vegas urbanized area. No use signing Las Vegas when most people think they are already in Las Vegas (even if technically speaking, they are not in the city but the urbanized area generally known as Las Vegas). According to NDOT mileage, the Las Vegas southern urban limit is at milepost 25.430, which puts it right around or just north of the Sloan exit (south of St. Rose Pkwy).