Investors may want to consider bracing for a weaker stock market performance over the next six months.According to Vanguard's Roger Hallam, it's prudent for long-term investors to have sufficient exposure to fixed income in this environment."Our outlook for the second half of this year is that growth will slow," the firm's global head of rates told CNBC's "ETF Edge" on Monday.Hallam predicts the labor market will continue to gradually cool while inflation rises. Hallam expects the Federal Reserve will ultim ...