This has been a contentious issue in Hungary as well, where lot of people have been vaccinated with Sputnik V last year. One of the Hungarian EU MEPs have therefore asked the EU commission if a dose (or two dose) of Sputnik V followed by a single dose of an EU approved vaccine (like Pfizer or Moderna) will make it valid for cross border EU travel, or not.
The answer was that it is indeed valid, and has to be accepted when crossing borders within the EU, meaning a dose of Pfizer that followed one or two doses of Sputnik V will indeed make you vaccinated(*) when you wish to cross borders within the EU. In short: a border guard that tries to check your full vaccination history can not deny you entry, even if they see that you only have a single dose of an EU approved vaccine.
However this is only true for checks when crossing the borders within the EU, or entering the EU itself from a 3rd country. Member states can still set local laws on who is considered vaccinated or not. For example you might still be considered unvaccinated by a member state and not allowed to enter a restaurant. Countries which have a vaccination mandate (like Austria) could still consider you unvaccinated. However other answers have already mentioned that any member state using the current EU Covid Pass to check vaccination status will unlikely to notice that your vaccinations include non-EU approved ones - but there's still a very-very small, but non-zero chance that you might face issues.
Also this only applies within the EU, technically any other country accepting the EU Covid Pass, but doesn't consider the Sputnik V vaccine valid (for example the UK) could technically still consider you unvaccinated on entry. As others mentioned this is again unlikely - border guards usually just have a glance on your pass, checking that it looks valid and is showing 2/2 or 3/3 doses. I personally have never seen a border guard neither in the UK nor in the EU checking whether the QR code is valid or not for example.
There's plenty of Hungarian sources for the announcement, but I also found an English one as well:
Travel of Those Vaccinated with Eastern Vaccine in EU Resolved
According to Ujhelyi’s announcement, Didier Reynders has made it clear to him that all EU Member States are obliged to accept EU vaccines administered for one or two eastern vaccines as a condition for entry without restrictions, i.e. without costly testing for the Hungarians concerned.
The MSZP MEP called the announcement good news, but also pointed out that the EU institutions have only made border crossing and entry free of restrictions, while it is always necessary to find out about individual restrictions within the borders of the member states (restaurants, public transport, use of social spaces, etc.), as these are the responsibility of the countries concerned and may differ from one another.
Also note that the current EU Covid Pass might change in the future as well. There are plans to add more information to it, like previous vaccinations, which could make it more visible that you had two Sputnik Vs before the Pfizer:
The EU Commission has recommended some limited amendments to the Regulation (...) Making sure that the correct number of COVID-19 vaccine doses is included in the certificates even when the EU citizen has received the doses in the different Member States.
Again this should have no effect on entering the EU / crossing the borders, but might affect your ability to use local services which require proof of vaccinated status. However it is still unlikely (given the relaxed verification done in most countries) that this is going to cause issues for you.
(*): do note, vaccine expiration still applies though, so if you only had two doses (single Sputnik V folllowed by a single Pfizer), and the second dose was more than 6 month ago you might still be considered unvaccinated until you get your booster.