Have you tried searching with different transliterations? Bulgarian is written with Cyrillic letters and German-speaking countries do not always use the same transcriptions as English-speaking ones. Transcriptions do also change over time, e.g. from Dimitroff to Schiwkow.
I have found two results on Google. One is a long Polish text that has the same names as your question (e.g. "Weselin Stojow", at http://weekend.gazeta.pl/weekend/1,152121,20687950,tak-zabija-rosja-w-1978-r-kgb-dostalo-dziwna-prosbe-trzeba.html ) and the other is a roughly contemporary German-language article that gives the murderer's name as "Stefanoff" and says there were only two people killed (https://m.spiegel.de/spiegel/print/d-40616193.html )
Edit: The relevant part from the Polish article looks reasonably similar to the relevant part of the article linked by the OP (according to Google Translate), minus the part where events from the mid-70s are moved to the mid-80s. I wonder if the confusion of two vs. three killed might be related to false passports and alias names (e.g. two bodies, but three names)?
Here is the relevant part from the Spiegel article:
Bald darauf schon exekutierte der Sicherheitsdienst an Ort und Stelle: Der Emigrant Stefanoff erschoß im Januar 1975 in Wien zwei Exil-Bulgaren, die sich auf Herstellung gefälschter Pässe und anderer Dokumente spezialisiert hatten. Nach dem Doppelmord flüchtete Stefanoff in die bulgarische Botschaft.
Vergeblich bemühte sich Österreich um seine Auslieferung. Stefanoff wurde nach Bulgarien gebracht, der Form halber zum Tode verurteilt, dann begnadigt und aus dem Gefängnis entlassen.
And a translation:
Soon afterwards the security service started executing [their enemies] in situ. In January 1975, the emigrant Stefanov shot and killed two exiled Bulgarians who had specialized in creating false passports and other documents. After the double murder, Stefanov fled into the Bulgarian embassy.
Austria's attempts to have him extradited were in vain. Stefanov was brought to Bulgaria, was pro forma condemned to death, then pardoned and released from prison.