What would be the airspace class when no vignette is visible on a US VFR sectional chart?
The vast majority of US airspace where no more restrictive airspace exists has class E starting at 1200ft AGL with class G below. You can see an area of class G up to 14,500 MSL just north of the TCS VORTAC near to Truth or Consequences, NM which is south-southwest of KABQ. There are several other areas in the 48 states as well. As long as the airport you are talking about "near airports with pink vignettes" is not under the pink vignette, then G extends to 1200 AGL. If it's inside the pink vignette, G extends up to 700 AGL. Of course, if it lies within a pink dashed line indicating class E to the ground at such a nearby airport, then the airport you mentioned does not have any class G above it either.

One other helper I use - if you see any federal airways nearby and there is no blue vignette indicating the presence of class G up to 14,500 MSL then you can usually safely assume it is class E above 1200 AGL if no other more restrictive airspace overlies the area. Federal airways are 4 nm wide on each side or the course centerline and always exist within controlled airspace (class E or better). These are labeled on a VFR chart as the light blue lines with V-number labels - also called Victor airways. As such, V 49 west of the M20 airport you mentioned would be an example and is pronounced "Victor forty-nine".

At M20 - the airport you ask about - the airspace up to 1200 ft AGL is class G, then class E up to but not including 18,000 ft MSL, then class A at 18,000 MSL up to FL600, then class E to space.