上級者が英語力（特に読解力）をつけたければ、語彙を増やすのが王道ですね。

（）、。

If advanced learners want to improve their English — especially reading comprehension — building vocabulary is the royal road.

Literal

Advanced-learner [subject-が] English-ability (especially reading-ability) [object-を] want-to-build-[conditional-ければ], vocabulary [object-を] increase [nominalizer-の][subject-が] royal-road [copula-ですね].

上級者 'advanced learner.' 読解力 'reading comprehension ability.' 語彙を増やす 'to increase vocabulary.' 王道 'the royal road, the orthodox/sure path' — originally a Confucian image of benevolent rulership, now a stock metaphor for the proper, well-trodden way to do something. ~ければ is the conditional of ~たい: 'if you want to.'