Party leader Santiago Abascal denounced the outgoing government in a speech to 12,000 supporters at a rally in Madrid.
He picked up on the government's controversial plans to remove the remains of the dictator Francisco Franco from a grandiose mausoleum near Madrid.
"The remains of General Franco are only an excuse," he said of the controversy.
"The aim is to rewrite history, the aim is to delegitimise the monarchy and the aim is to topple (King) Felipe VI."
The legacy of Franco, who ruled with an iron fist following the end of Spain's 1936-39 civil war, still divides Spain today.
Abascal also attacked the conservative Popular Party and the liberal Ciudadanos, who he said were ready to ally with the socialists to end the political impasse.
Ciudadanos leader Albert Rivera on Saturday appeared to shift his position by saying he would consider helping Sanchez's socialists back into power after the next elections.
Popular Party leader Pablo Casado has raised the possibility of a grand coalition including the socialists if his party wins the elections, which according the latest polls seems unlikely.
The new elections, only months after April polls, have been called because Sanchez's socialists, though Spain's single largest party, were unable to negotiate a ruling coalition with rival parties.
Practically unknown last year, Vox in April became the first party to figure as a far-right political force since Franco's death in 1975.
The latest polls put Vox at around 10 percent, similar to the level that won them 24 out of 350 seats in April.
Shortly before Vox's Madrid rally, four members of the feminist activist group Femen chained themselves to the venue, the Palacio Vistalegre, in protest, denouncing Vox as fascists.
November's parliamentary elections will be the fourth in four years.