As far as I remember it is pronounced like ['vaɪəʁˌʃtræs] but not quite sure about it (specially whether it is a "ʁ", "ʀ" or "x" in the middle).
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Usually, the "r" in the middle is vocalic, that is, ['vaɪəɐ̯ˌʃtʁas] or ['vaɪɐ̯ˌʃtʁas]. Using [ʁ] instead of [ɐ̯] is also fine (some might claim that this is the "official" pronunciation). I would not use a trill here.
The second "r" may be a fricative [ʁ] or a trill [ʀ], [r], depending on your dialect. The last vowel is usually pronounced short by mathematicians, but a long vowel would also be possible. It's certainly not [æ].
It is pronounced ['vaɪ̯əʁˌʃtʀas], ['vaɪ̯ɐˌʃtʀas] or ['vaɪ̯əʀˌʃtʀas], depending on the dialect, under the assumption that the a is a short vowel. According to a lot of old orthographies, ß could be preceded by both, a short and a long vowel. This does not exclude ['vaɪ̯əʁˌʃtʀaːs] and similar, however, having studied Mathematics in Germany for some years, I have never heard anyone claiming that he would be pronounced like this.
I do not see any reason to assume the pronunciation ['vaɪ̯əʁˌʃtʀæs] or ['vaɪ̯əxˌʃtʀas].
Not being a mathematician, my inclination was to pronounce it
with the sound of the third syllable equivalent to the
A video recently added to Youtube confirms my intuition by showing the phonetic rendering –
Cross-checking with a teaching video also on Youtube, I find my confusion resolved in favor of my original intuition. The author, presumably a mathematician, pronounces the name exactly as I would expect: