Given Information:
- The vendor bought bananas at a rate of 6 for 5 rupees.
- The vendor sold bananas at a rate of 4 for 3 rupees.

Calculating the Cost Price:
To calculate the cost price, we need to find out how much the vendor paid for each banana. Since the vendor bought 6 bananas for 5 rupees, the cost price of each banana can be calculated as follows:

Cost Price per Banana = 5 rupees / 6 bananas = 5/6 rupees

Calculating the Selling Price:
To calculate the selling price, we need to find out how much the vendor received for each banana. Since the vendor sold 4 bananas for 3 rupees, the selling price of each banana can be calculated as follows:

Selling Price per Banana = 3 rupees / 4 bananas = 3/4 rupees

Calculating the Gain or Loss Percentage:
To calculate the gain or loss percentage, we can use the following formula:

Gain or Loss Percentage = (Selling Price - Cost Price) / Cost Price * 100

In this case, the selling price is 3/4 rupees and the cost price is 5/6 rupees. Substituting these values into the formula, we can calculate the gain or loss percentage as follows:

Gain or Loss Percentage = ((3/4) - (5/6)) / (5/6) * 100

Simplifying the equation, we get:

Gain or Loss Percentage = (-7/24) / (5/6) * 100

To divide a fraction by a fraction, we can multiply the first fraction by the reciprocal of the second fraction:

Gain or Loss Percentage = (-7/24) * (6/5) * 100

Simplifying the equation further, we get:

Gain or Loss Percentage = -7/4 * 100

Therefore, the gain or loss percentage is -175%.

Conclusion:
The vendor has incurred a loss of 175%. This means that the selling price of the bananas is 175% less than the cost price. In other words, the vendor has sold the bananas at a price that is 175% lower than what he paid for them.