English idiomatic expressions aren't easily translatable in French... I thought of "vertu ostentatoire" but a single word would be good.
When "vertue-signalling" is used pejoratively to despise some action or attitude, such action could be named a tartufferie (it can also be spelled tartuferie) in French.
The noun comes from Tartuffe, a famous character from one of Moliere's plays.
I must admit that it is far from perfect to propose a noun instead of an adjective (edit: this caveat is void, as Christophe pointed in a comment: "virtue signaling" is a noun in English too !), but the meaning conveyed has the same nuance : smg is done for the show, to frame oneself as virtuous.
One must confess that after what the Americans themselves made of the concept it'll be very difficult to find a translation that means really something in French; this is indubitable from what can be gathered in a Wikipedia article;
"Virtue-signalling" is also used as a pejorative term, denouncing empty acts of public commitment to unexceptionable good causes such as changing Facebook profile pictures to support a cause, participating in the Ice Bucket Challenge, offering thoughts and prayers after a tragedy, celebrity speeches during award shows, and politicians pandering to constituents on ideological issues.
The term was popularised by James Bartholomew in an article in The Spectator on 18 April 2015 to mean "public, empty gestures intended to convey socially approved attitudes without any associated risk or sacrifice".
Lexicographer Orin Hargraves says that the term stems from social media, which removes barriers to broadcasting sentiments. Hargraves links the term to the "shaming" category of neologisms, such as "prayer-shaming", which can have an opposite meaning to virtue signalling. Merriam-Webster editor Emily Brewster described it as an academic-sounding counterpart to "humblebrag", a term coined by Harris Wittels in 2010.
Signalling virtues such as environmental responsibility has been associated with economic decisions of consumers, such as buying "green" products and other forms of conspicuous conservation.
Criticism Jane Coaston of the The New York Times notes that in using the term "virtue signalling" one is "trying to signal something about their own values: that they are pragmatic, appropriately cynical, in touch with the painful facts of everyday life". In The Guardian, David Shariatmadari argues that this makes it "indistinguishable from the thing it was designed to call out" adding that it is "smug posturing from a position of self-appointed authority." Neoliberal political theorist and economist Sam Bowman, criticized the term claiming that "virtue signalling is hypocritical. It’s often used to try to show that the accuser is above virtue signalling and that their own arguments really are sincere".
Adam Smith Institute Executive Director Sam Bowman opined that the meaning of the term popularised by James Bartholomew misuses the concept of signalling and encourages lazy thinking. In The Guardian, Zoe Williams suggested the phrase was the "sequel insult to champagne socialist" while fellow Guardian writer David Shariatmadari says that while the term serves a purpose, its overuse as an ad hominem attack during political debate has rendered it a meaningless political buzzword.
I will assume therefore a rather neutral point of view in the use of the term.
In the word "signalling" aren't found the ideas associated to hypocrisy and that rules out such words as "hypocrite" and "tartuffard";
The word "signalling" implies essentially the idea of "advertisement", without any implication as to honesty, sincerity or the depth of the convictions; it is not especially a disapproving term, no real moral judgement on an attitude; that makes the following terms,
- ostentatoire (tapageur, voyant), de m'as-tu-vu,
terms that force a possible aspect that is not stressed originally.
It is not descriptive either of the strictness of an attitude but only of the manner in which an attitude is made known to others; therefore I should discard also such terms as "pharisaïsme";
- (TLFi) pharisaïsme : Attitude de celui/celle (caractère de ses actes, de ses idées) qui, croyant incarner la perfection morale, porte des jugements sévères sur l'attitude ou le comportement d'autrui.
A translation that seeems to remain within the limits of what appears to be intended as most common place would be "démonstratif(ve) dans sa vertu"; however this is a term that supplies only a basis and according to the case considered, adjustements may have to be made so as to convey the idea idiomatically.
(TLFi) [En parlant d'une pers., de sa nature, ou de ses sentiments] Qui manifeste ses sentiments/qui se manifeste par des signes extérieurs, qui est expansif.
- They appeared as virtue signalling individuals at best.
- Ils avaient l'apparence au mieux d'individus d'une vertu démonstrative.
- Ils avaient l'apparence au mieux d'individus vertueux d'une façon démonstrative.
- Ils avaient l'apparence au mieux d'individus démonstratif dans leur vertu.
For instance I would consider translating "his discourse, accessorily virtue signalling, did not convince" as
"son propos, démonstratif dans la mise en avant de sa vertu, n'a pas été convaincant" or
"son propos, empreint de notes démonstratives sur sa propre vertu, n'a pas été convaincant".
- He reproached him with what he called his virtue signalling but lackedaisical action.
- Il lui a reproché ce qu'il a appelé son action démonstrative dans la mise en avant de sa vertu mais indolente.
That's a tough one! What about rayonner de vertu, irradier la vertu, puer la vertu, suinter la vertu, des parangons de vertu, les pères-la-vertu, les mères-la-vertu, les pue-la-vertu? I think you have to convey the put down that virtue-signalling or virtue-signaller is in the minds of those that use it.