It is one of the muscles situated next to the vastus medialis and it is called like this because it has an intermediate position among other muscles.

The vastus intermedius lies between the vastus lateralis and medius and beneath the rectus
http://www.ehow.com/list_7389602_primary-movers-lower-extrem...

Vastus Intermedius (frustrator) (Fig. 14.3) The vastus intermedius muscle is a " frustrator" because it develops many TrPs that cannot be palpated directly
http://www.google.com/search?q=google &rls=com.microsoft:en-...

The Vastus intermedius (Cruraeus) arises from the front and lateral surfaces of the body of the femur in its upper two-thirds, sitting under Rectus Femoris and from the lower part of the lateral intermuscular septum
http://en.wikipedia.org/wiki/Vastus_intermedius_muscle

Vastus medialis
‘…The vastus medialis (aka vastus internus), often called the 'teardrop' muscle,…’
http://en.wikipedia.org/wiki/Vastus_medialis


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Note added at 4 hrs (2011-03-07 01:22:41 GMT)
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To decide which term to use you will check the prospective reader of your translation. If you address doctors then you may keep the latin definition. If, however, you address lay readers ( advertisement, manuals ) do not forget to include the muscle nickname in brackets. Athletes are very likely to know it as frustrator