A medium pace bowler with no extra wicket taking dimension of height, bounce, swing or seam cannot be successful at Test level. Moreover, a medium paced bowler is often reliant on pitch conditions to aid them.
Chaminda Vaas operated at 115-120kph toward the end of his Test career, while operating with the old ball (bowled at 130kph with the new ball). He had success in West Indies, Sri Lanka and Bangladesh, all of whom offered slow, low tracks which exemplified the lack of pace and made it difficult for batsmen to score off him and also go back to deliveries that may keep low. When he lulled the batsmen into this defensive shell, it caused complacency in the batsmen's strokeplay, allowing him to take wickets with even the slightest deviation off the straight.
Someone like Jacob Oram is quite the opposite. In New Zealand, where the pitches are often moist, the ball can skid through and he can bowl slower than full capacity, at around 120kph and still pick up wickets because the pitches make it seem as 130kph and he can cash in on the extra accuracy he can provide while slowing down. Oram also has the height which adds intangible pace to his deliveries (intangible in that it is not registered on the speed gun).
The key is that both these bowlers are operating at well below their full capacity of pace and so those who bowl with the same effort as any other fast medium bowler (but at 120kph) will tend not to have this extra accuracy and therefore will struggle at Test level.