Lenin was not a member of a purely vanguard Marxist party either; the RSDLP contained genuinely vanguardist elements, but also thoroughly reformist ones that agitated for better labor conditions but downplayed and even abandoned their struggle against the repressive Tsarist state (i.e. "Legal Marxism"). Organizations such as DSA in the States and Die Linke in Germany are similarly "Legal Socialist" or "Legal Social-Democratic" parties; unlike pure reformist parties (SPD, British Labour) they openly criticize capitalism, but are afraid to openly challenge the liberal constitutional order for (admittedly valid) fears of being criminalized. But there are elements that are worth engaging with, just as Lenin did. There is no shame in splitting to protect the revolutionary faction from revisionists/liberals, but it's also not ideal.

I would suggest to engage in the party's associated organizations (youth orgs, student orgs). Look for what interests you, whether it is community/labor organizing, direct action, education, mutual aid, electoral campaigns, or even just showing up at a variety of the above and volunteering/baking cookies or something. If the party's organizations have an open political culture, there are bound to be activists you have affinity for. Develop your politics with those comrades, participate in readings/campaigns with your circle, and maybe evolve into a revolutionary faction. Only by demonstrating that the revolutionaries of the party are more capable of leading the proletariat than reformists can Marxist-Leninists in a left party gain hegemony over the broader social-democratic movement.