The Vaishya can take only one wife. He can take a second wife from the Shudra caste. Such is the practice, no doubt, but it is not sanctioned by the scriptures.
एकैव हि भवेद् भार्या वैश्यस्य कुरुनन्दन। द्वितीया तु भवेच्छूद्रा न तु दृष्टान्ततः स्मृता॥