Like others have already said, it may cause an immediate dip due to a large and sudden move in shares for that particular stock. However, if there is nothing else affecting the company's financials and investors perceive no other risks, it will probably bounce back a bit, but not back to the full value before the shares were issued. Why? Whenever a company issues more stock, the new shares dilute the value of the current shares outstanding, simply because there are now more shares of that stock trading on the market; the Earnings Per Share (EPS) Ratio will drop since the same profit and company value has to be spread across more shares.
If a company is valued at $100 dollars and they have 25 shares outstanding, then the EPS ratio equates to $4 per share (100/25 = 4).
If the company then issues more shares (stock to employees who sell or keep them), let's say 25 more shares, then shares outstanding increase to 50, but the company's value still remains at $100 dollars. EPS now equates to $2 per share (100/50 = 2).
Now, sometimes when shareholders (especially employees...and especially employees who just received them) suddenly all sell their shares, this causes a micro-panic in the market because investors believe the employees know something bad about the company that they don't. Other common shareholders then want to dump their holdings for fear of impending collapse in the company. This could cause the share price to dip a bit below the new diluted value, but again if no real, immediate risks exist, the price should go back up to the new, diluted value.
If EPS was at $4 before issuing more stock, and then dropped to $2 after issuing new stock, the micro-panic may cause the EPS to drop below $2 and then soon rebound back to $2 or more when investors realize no actual risk exists. After the dilution phase plays out, the EPS could actually even go above the pre-issuing value of $4 because investors may believe that since more stock was issued due to good profits, more profits may ensue.
Hope that helps!