You have to keep in mind that the vocabulary size is not the only factor which will affect the LSA results.
In my experience a lot depends on the inflectional properties of a language. English is very LSA-friendly in this respect as it has virtually no inflection. Some Romance languages on the other hand still have pretty rich inflection so that having different strings for similar meanings will 'confuse' LSA unless you have a very good stemmer/lemmatizer.
If your primary concern is disambiguating the context you can try out some other techniques for discovering semantic structure. E.g. topic models (Latent Dirichlet Allocation; e.g. Blei et al., 2003) deal with homonymy much better.
If, theoretically, there were no other factors involved in addition to vocabulary size, I would expect that the ratio between vocabulary size and corpus size would matter more than vocabulary size alone.