Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Vishakha - Vishakha is nakshatra of counselors and guides. It is also nakshatra of split or partition or different direction. For more information on Vishakha Nakshatra, please check this link - https://www.astrosaxena.com/vishnaksh.
Libra - As Vishakha is a part of Libra, Libra sign and its representations also become important. Libra is the 7th sign of Zodiac Belt, hence it represents the energy and things related with 7th house of horoscope, i.e. Market Place, Business, Partnerships, Relations etc. Libra is sign of Balance, i.e. striking the Balance between 2 parties/ideologies or even at personal level between our own dilemmas. It is sign of Peace and Diplomacy. And this is what we are seriously looking for after the conflicts and disagreements of Virgo, i.e. Peace to prevail. At personal level, Libra is seen as sign of Relationships. Libra is consisted of 2 and half Nakshatra named Chitra, Swati and Vishakha. Libra's lord is Venus only.
Scorpio - As Vishakha is a part of Scorpio too, Scorpio sign and its representations also become important. Scorpio is the 8th sign of zodiac belt, hence it represents the things and energy related with 8th house of horoscope like Sudden Events, Secrecy, Dark World, Occult Science, Research, Inheritance, Serving Others, Taxes etc. Besides these things sign of Scorpio also represent transformation, means changing the whole nature upside down. This is one sign which is favorite of every Astrologer, because it has maximum secrets, so every astrologer gets maximum scope of researching. This is sign about which no one can tell with precision that what will happen to the planets in it, as it is sign of secrecy itself. Wherever Scorpio falls in your chart, you may have had biggest challenges in making things related with that house more stable. Scorpio is also one of those unique signs which has 2 rulers, i.e. Mars and Ketu. In Western Astrology, Pluto also co-rules Scorpio. All this makes Scorpio a chaotic sign. Scorpio consists of next set of 2 and half nakshatras, i.e. Vishakha, Anuradha and Jyeshtha.
Jupiter - As Jupiter rules Vishakha, Jupiter's position and dignity is important to know about the overall functioning of any planet in Vishakha.
Venus - As Venus rules Libra, Venus position and dignity is important to know about the overall functioning of any planet in Vishakha/Libra.
Mars-Ketu - As Mars-Ketu rules Scorpio, Venus position and dignity is important to know about the overall functioning of any planet in Vishakha/Scorpio.
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Vishakha Nakshatra – Mercury is in friendly sign in Libra but in enemy planet's nakshatra. So, it is an average position of Mercury. Its basic interpretation is a counsellor or teacher but Vishakha spreads from Libra to Scorpio and here lies the basic difference in interpretation. As Libra is sign of peace and diplomacy, it shows a counselor who is trying to create a peaceful environment between two warring parties. as Libra is sign of relationship and vishakha represents a split, it shows a relationship counsellor or a divorce lawyer too. But when Mercury is in Vishakha/Scorpio, it is in enemy sign plus basic nature of Scorpio is cruel. Mercury can take shape of an occult counselor here as Mercury and Scrpio both represent occult side of life. Otherwise also, it is a good position for any research oriented work. If Mercury is a relationship oriented planet for a person then it is troublesome position because Vishalha itself represents split.
Role of Jupiter – Jupiter placement is important to guide Mercury in right direction, being Vishakha's lord.
Role of Venus - Likewise, Venus placement is important to guide Mercury in right direction, being Libra lord.
Role of Mars-Ketu - Likewise, Mars-Ketu placement is important to guide Mercury in right direction, being Libra lord..
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Vishakha. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Vishakha can work in a chart.
Hope this helps. Please post any comment or query you may have.